I understand that the 500 was developed in response to requests from the Garmin Professional Cycling Team. They wanted a unit that was smaller, and had no need for maps (that's why they have course officials and team cars.) So it's a compact unit that shows lots of data with very limited route capability. Also has the ability to communicate with power meters.

I have an old 305. Works fine, but if it ever goes bad I would get the 500, not the 705. I ride locally on known routes and have no need to follow maps. As a basic bike computer the Garmin has a great range of data and fully customizable screens. So I can have elevation, grade, lap time, etc., all of which I use. And I can upload data from my Garmin and have a record and map of all my rides.

I'm really a fan of the Garmin products.