The edge 500 is definitely a capable little bike computer, and they're fairly inexpensive now. the 800 is a nice step up with some extremely irritating navigation quirks. the "10" models (510, 810) are newer and apparently less bug free than the average user would like. I have a 500 and a 800 fwiw and prefer the 800 despite the aforementioned irritating navigation quirks. you can get a speed and cadence sensor (optional) for each bike and then before your ride, just identify which bike you'll be on and off you go. Garmin can usually store at least 5 bikes.
I have seen some positive things lately about magellan's bike computers. The cyclo 500 series is apparently pretty good, and speculation (by me) is that Garmin rushed out their edge 1000 because of it.
Whever you do, NEVER buy Garmin's latest and greatest. They never seem to work correctly out of the box until at least a few firmware updates. And they're priced sky high as well.