I'm going to have to try what Roady did and record with both IpBike and Strava sometime.
Did my commute home with IpBike with my Garmin GSC10 (uploaded to Strava) and the Strava app.
IpBike (consider this the most accurate?): http://www.strava.com/activities/204877382
Strava (via Phone GPS): http://www.strava.com/activities/204878619
Distance is different. Average speed is different. Moving time is different (I stopped/started apps within seconds of each other).
Only slightly, but on my 4.4mile short commute it shouldn't be that different? I guess Strava calculates speed differently and distance travelled via GPS which might not be that accurate?