I agree with modman concerning the 500. I have a 510 and it's awesome; super easy to set up and use. but truthfully, I probably would have been just as happy with the 500. I don't think I use many (if any at all) of the additional features. The review below has a section that goes over the differences between the two. It's a pretty lengthy and comprehensive read, but if you're dropping that much cash it might be worth at least a skim.

Garmin Edge 510 In-Depth Review | DC Rainmaker