Go look in the SUPPORT section on the STRAVA website...it explains why your Edge 500 data differs than what Strava calculates. Its a combination of how Strava collates all of the data, coupled with the fact that STRAVA reacts strangely if you have AUTO PAUSE enabled on your device.
Bottom line, I use Garmin Connect for my own personal training database, but I use Strava for the interaction and challenges with my friends.