Hi Faema!
Though still in the early stages, your upcoming trip sounds fabulous, and I hope you'll keep us updated as your plans develop.
Looking at your question
from the frame of my own experience, a few things jumped out at me when you said...
I rather take the slower routes in the hope that they are more scenic with less traffic and more opportunity to wild camp.
...this is more the sort of thing I do, where I am away from any sort of consistent power sources for a good long while. Just as Richie and Pete (Il Padrone) have alluded, gadgets like GPS take power, and they don't last very long on a set of batteries. The solution is to charge the batteries for such gadgets as you ride, either by dynohub, solar, buffer-battery (charged from mains power) or some other means.
This all goes to show the importance of taking paper maps of some sort as a backup. batteries fail, and then where would you be? I think a good compass with adjustable declination is essential, too -- even a cheap compass goes far toward orienting one for sure (as a check, even if you have a very good sense of direction and magnetic orientation as I do; not something you want to leave entirely to "feelings"). This is the route I go, powering-on the GPS for just a few seconds at a time for a quick check of my position against my paper maps or as a check against the built-in maps. Even traveling to a previously marked waypoint, I use the GPS this way to save juice. The paper maps and compass are my proven fallback, as there isn't a living soul to ask directions from where I go.
I use a Garmin Oregon 400T GPS with touchscreen, 3-D colored/contoured maps and downloadable satellite maps that can overlay any existing or purchased Garmin-format maps. I run an 8G micro-SD card (a spare in reserve, depending on where I'll be touring), and typically have it loaded with the topo maps for all of North America (Canada, US, upper part of Mexico) or all of Europe and park maps as well. I then have an automobile (roads) profile I overlay on the topo maps that includes the city maps for North America or Europe, with another for Benelux, or the fietspads for all The Netherlands. The GPS maps costs add up; at full list, the GPS and maps totaled USD$1,200 at the time I got it in 2010 (updates have been free, thank goodness, and I got the GPS on sale at a wholesaler for half price). It is the map costs that are so high.
There are ways to save on the cost of paper maps...
One of the great things about Garmin GPS units is they not only offer lifetime updates, they have two really good free mapping programs that will run on a PC -- MapSource and BaseCamp. The latter is really handy for cyclists, in offering tilt-contour capability that brings contour lines to life so you know exactly where that mountain pass is, or how steep that big climb really will be. I plot a course in either of these two applications, then transfer the routes and waypoints to my GPS. It is a two-way street, so you can take a trip and then transfer your waypoints or tracks to the PC map upon your return. These maps can then all be printed out and so comprise your "paper" maps without spending an additional penny.
This has worked out very well for me in the more remote places I generally travel here in North America. However, since I start these journeys from my front door, there's a lot of road mileage to get out of the way as well. For that portion of my journeys, I use Microsoft Streets & Trips, DeLorme Street Atlas and Topo-series software, GoogleEarth, GoogleMaps, and BingMaps. I print what I need to take with me, and -- since inkjet inks are water-soluble -- make sure I put them in two zip-top bags, reversed top-for-bottom to keep water out. They're stored in my waterproof Ortliebs or put in my waterproof Ortlieb map case when in-use. Each of the above allow routing overlays and GPS waypoints to be printed on either a map or (in the case of the online services) a satellite photo overlay. I found the above combo to be very helpful in my Great Basin tours, and correlated the map data with over 150 years of state hydrology reports and well registrations so I would have a very good idea where potable water was "supposed" to be. In fact, overlaying the satellite imagery on the well data showed persistent spots of green vegetation, even in otherwise arid areas, and I was able to find non-alkali "ground seeps" and even small springs in the very areas where they "should" have been. Yay!
GoogleEarth/GoogleMaps StreetView is tremendously useful fir planning in advance at home to get a look-see at confusing intersections or even to read street signs -- which often differ in real life compared to map designations (I often find local names are used in place of County Road numbers on maps, for example).
My bigger trips require 3-4 months of fairly intense planning, and are worth it in peace of mind and navigational ease. If I decide to deviate from my planned course, then I still have a baseline to return to. Before leaving, I have printed and electronic packets showing my itinerary, route, attached maps, and so forth that can be forward to SAR teams if I fail to check in after a predetermined absence. When I can connect, I call home my GPS coordinates to friends or family who are tracking my route, so they can see where I am, will now where I am headed, and can track my progress accordingly in the event something goes really wrong. This may not be a need for you; I go alone and to really remote areas where I am unlikely to see or make contact with anyone else for long periods. A mishap in such circumstances could quickly become serious, so I take the precaution of making sure someone knows where I "should" be and will take action on my behalf if they fail to hear from me. These trips are supposed to be fun, and I do wish to take more in the future and not endanger others or incur a huge expense in the effort to find me. <-- This last has some major implications. More and more cash-short American search efforts are being billed to the missing recreationalist, and search costs often exceed USD$100,000. I do not want to incur that sort of bill. I also carry insurance to cover the USD$22,000 average cost of a helicopter evacuation, which can otherwise be billed direct to the rescuee. Many of the roads I go on are impassable by four-wheel vehicles for much of the year, and accessible only by high-clearance four-wheel-drive in better weather. Air searches and evacs are the most common form of extraction in those places for a number of reasons.
One of the problems I run into which you might not is cell-phone connections. I am usually so removed from any towers that the phone is rendered useless for voice or text communication, and I deliberately run an old 3G CDMA Kyocera SE47 "dumb" phone because it connects far better than my nearly current Motorolas. Even so, I am often without comms for a day or two and then get it only with line-of-sight if I deliberately climb a mount so I can get a connection -- if I am lucky. Smartphones with GPS often need tower-triangulation for the GPS coordinates to work properly. If you're out of range, the GPS may not operate or be as accurate as you would wish. For this reason, for trips away from reliable cell coverage, I would suggest a standalone GPS rather than a smartphone with GPS capability. Returning to why I chose the Garmin Oregon 400T...I found a GPS oriented primarily toward hiking rather than cycling or automotive use better suited my needs for back-of-beyond use. Each GPS on the market seems to be optimized for a particular use, with other uses secondary. It worked out best for me and the kind of riding I do to have a GPS oriented toward 3-D topo use first, with on-road capability playing a supporting role. As such, I don't have uploadable fitness data and such, but I can plug in a Garmin bike pickup if I wish and I have more navigational capability than the biking-oriented GPSs. Compared to the car-oriented GPSs, I lack voice prompts and audio capability, but I can read and see the turn and real-time routing indications and have full Topo data with satellite imagery overlay capability.
One of the handiest uses for GPS is what I call "casual navigation" while on-tour. For example, I prefer to stealth-camp, placing my tent and bike below sagebrush height. I locate my latrine away from camp for a number of reasons, and this can render my camp invisible from 30 meters away. I mark the camp with a waypoint when I arrive, and take the GPS and phone with me when nature calls. It would be tragic to find myself lost within stone's throw my (invisible, unseeable) camp and find myself without food, water, or shelter because I could not find it again. Waiting till daybreak to see my tracks wouldn't help, as shoes leave no marks on sun-baked playa. Often, there may be no mountains or other outstanding geological features in sight, and one clump of sagebrush look like the next. I can easily imagine such a situation, and don't wish to find myself in that scenario.
Most modern GPS units can be downloaded with commercial waypoints as well, and these can be really handy. Imagine keying in "Lodging" and having the names, addresses, and contact phone numbers of hotels and motels appear with waypoint icons on your screen. Same for "Geologic Features", "Stores", "Cities", and myriad other places. Using this feature has saved me a lot of time. I found the "Lodging Finder" particularly helpful when a convention was being held in a destination town and hotel after hotel closed due to full bookings. Without having to find a phone booth and directory listing, I just called each on the list till I found one with a vacancy, made and confirmed my reservation, and cycled in a relaxed manner to my night's lodging using my waypoint routefinder. Magic!
Pete (Il Padrone) put it very well when he wrote...
Despite all the technology available, there is still a serious role for the humble paper map. GPS is great for accurate location of your position and to measure distances to other places. They are also great to upload your route maps for others. However they do not give a good overall idea of your surroundings and the options for travel routes or other destinations. The paper map is the best way to get a 'global' view of your surroundings.
Hope this helps.
Best,
Dan. (who would simply be lost without his nav aids...)