I like the cheapest Sigma, the one on my bike being the wired Sigma 509 which replaced a lost Sigma 506 (replace the O-ring fasteners with tiewraps, as Dan says). The reason I don't have a wireless bike computer, even though now they're pretty cheap and even appear as specials at the supermarket, is the usual one: interference with and by other electronics, like my heart rate monitor. A wired bike computer is just more reliable.
The Sigma 509 offers the basics: Current speed reported without delay, trip distance, trip elapsed time, accumulated total distance over all rides, current time. The single accessible button, which runs the full width of the 509 and is easily operated with thick gloves, zeroes the trip distance and time, chooses whether to display one of the latter four functions permanently (current speed is always displayed) or to alternate them automatically. All numbers are instantly readable at a glance and you soon learn to glance down, up at the road, down again, until the number you want appears; you get the information you want much more quickly than it takes to read this phrase, and there is no possibility of confusion because each piece of information is differently centred on the bottom of the display. The Sigma 509 is water resistant to a pretty high degree (in about thirty years with Sigma bike computers and heart rate monitors, I've never lost one to water ingress) but there is a price for it: the buttons under the computer for setting it up are awkwardly rubber-covered; fortunately you access them only when you first set up the 509.
The 8FUN C965 information centre for the electric motor controller on my bike offers all the information the Sigma 509 displays, albeit less conveniently accessed and not nearly as readable at speed as on the 509, but it lacks one gravely important facility.
See, on the Sigma 509, which I remind you is the cheapest in the Sigma line, when you set it up initially, you tell it how far the bike, or the gearbox on the bike (say your Rohloff) has already run, so that the total accumulation of mileage starts where the previous bike computer left off. You therefore have no excuse for missing service intervals. This is such a useful facility that I kept the Sigma on my bike even after it became clear that the C965 is an acceptable substitute for the information the Sigma 509 offers. I should add that the ease of accessing the Sigma information -- it's automatic if you don't switch off the default scrolling mode -- and its clarity for reading at a glance are also factors in my decision now that I'm not likely ever to ride as much as the 3000m/5000km in a single year, which is the service interval parameters on the last service-requiring component on my bike.
With both the Sigma 509 and the 8FUN C965 installed on my bike to facilitate a choice (it is possible to run the motor without the C965 reporting facia), I found I was taking information offered in common almost 100% more from the 509 than the C965 despite the fact that the C965 is directly in my line of sight and the Sigma 509 has been repositioned to beside rotary gear control on the handlebar grip end. I've left them both on the bars because running the motor without the C965 is slightly inconvenient. BTW, the C965 confirms the Sigma 509's impressive accuracy previously measured between mileposts and against electronic watches; in fact, the 509 updates current speed a fraction of a second faster than the C965.
The Sigma 509 bike computer is a proven instrument. Highly recommended for its simplicity, clarity, longevity, pure use-friendliness, low cost, and outright value.
