I have no experience of the Alfine but I have both the standard and premium Nexus models. IIRC, the premium model has better bearing and seals (said to be "Ultra-level"). I wrecked both my Nexus boxes before 5000km/3000m. But there are hundreds of thousands in commuter service in The Netherlands and elsewhere with untold miles on them, so maybe I was unlucky or maybe I'm just a masher-wrecker. I looked into the Shimano oil dip service kit a good 12 or even 15 years ago, at which time it cost £38 per service, half the price of a replacement box -- no surprise then that the thrifty Dutch would rather ride the box into the ground, since it seems to me even-steven whether you get something near the design life of the Nexus (Premium?) of 50,000km/30,000m or wreck it before the first service as I did, regardless of whether you service the thing.
One of my Nexus bikes has the IM70 or maybe IM75 roller brakes and I found them "superior" to disc brakes, that is, if you value very sharp, sudden and unprogressive brakes. I don't, so I hated them, though I understand their advantages, for instance in the wet, and the safety of their instant power in commuting crushes. They too need special grease from Shimano, and the hole through which you feed the grease is stopped only by a tiny rubber plug whose MTBF I estimated at 5 removals and insertions -- mine were lost before then. The late model roller brakes are exceedingly powerful.
I discovered is an advantageous application of the earlier, limper roller brakes. I used an IM40 on the rear of a bike braked by a disc in the front to give a measure of anti-blocker (ABS) braking so that the rear wouldn't come around on me at one intersection on a regular ride where I often braked hard at a T-junction at the bottom of a hill.
If it sounds like I'm not enthusiastic about the Nexus, that's the wrong impression. You have to evaluate the Nexus in the light of its cost, for instance in comparison to the Rohloff. In that perspective it is a genuine bargain -- and especially this time of the year when you can buy a complete Nexus box spoked into a desirable rim complete with fitting kit on German Ebay for fifty Euro if you're lucky, and nobody pays over a hundred Euro; this is the result of market pressure from manufacturers disposing before Christmas of last year's stock of wheels never built into bikes.
On Martin's question of whether the Premium feels different to the standard version, yes it does, a bit tighter, but not necessarily smoother. Maybe the standard model has looser tolerances, but mine didn't force it's punctilio on one as my Premium did. But my Premium might have been specially selected, or perhaps even blueprinted, because it came on a prototype bike which also boasts wheels built by Keith Bontrager, or so I was told; the wheels are superb. Didn't make it last any longer though.
For those converting a bike to a Nexus box, it is very important to buy the fitting kit with the Nexus gearbox; either one without the other is worthless. And make sure the fitting kit is utterly complete, right down to the fitting on the chain stay which blocks the gear cable sheath so the cable can do its job. The manuals are on the net, hundreds of pages, so make a list and check that SJS has everything you want to order, or failing SJS, Petra Cycles, which is (or was -- it's over ten years since I bought Nexus parts) a well-known Nexus specialist.
The infinitely variable transmission NuVinci is reputed to be a sturdier gearbox than the Shimano, even a competitor for the lowest level of Rohloff capability, and it is available with an automatic gearbox, which I would consider a plus, since the automatic Nexus (Shimano's Smover system of the full featured Di2 of a fully automatic self-shifting gearbox and active suspension) was a huge success for me, cutting substantial, noticeable chunks off my familiar daily rides.
