I've been using strava on Android for some time and have recently purchased a Garmin 800.
Having recorded some rides on both devices, i find that distance-wise accuracy on the Android is pretty good (+/- 100 meters on a 30km ride), with segment times very much inline. There can be a significant difference in elevation readings though.
You can store 'courses' on your garmin, which are akin to strava segments + timing data, and once loaded, use the virtual partner feature on garmin to show your deficit / lead over that 'course' in real time. I have found that feature quite useful to target PRs. I believe its also available on the 500.
*have the dashboard data on display in real time.
*Seeing your segment times stacked up with HR data (gets me thinking about how i might use the target HR zones to train better)
*Accurate weather readings over the course of the ride
*doesn't drain your battery as already mentioned
Its early days yet (I've only had the unit 2 weeks) but on balance definitely happy with the purchase, even without having started to use some other available features on the 800.