Most low-endy (say, sub-$25) cyclo-computers work just fine and are more or less equivalent. When you get above that, you're mostly paying for features like wireless and/or cadence. Or maybe a big display with lots of info. Personally, it's mainly a matter of getting the functions I like. In addition to mph and odometer, a trip meter and ride time are a must, and I really like a current speed above/below ride average indicator and 2 wheel size option (I switch between tires a lot).
Aside from that, the mounting and pickup HW is the determinant. I just got a cheap-o Sigma that mounts using (primarily) heavy duty rubber bands. I no like. I have a Cat-eye Mity-3, which I like pretty well, but the pickup is this long protrusion that just sticks out asking for something to bust it off. Which eventually happened and I had to replace it.