Unless you are able to precisely track the tangents on the Map Your Route feature to that of the app, there will be some discrepancies. As to which one you should follow--I tend to go with the GPS device--HOWEVER, when I run with my Garmin and my Nike + app, they can vary by as much as 1/4 of a mile. But I don't let that bother me--it's all relative. Because our bodies do not have odometers the distance covered is the distance covered.
BTW, it's even been reported that Garmin and other GPS device are not as accurate as many of us would like to think. Just use it as a tool and try not to get to concerned about accuracy.
Edited by: SP_COACH_NANCY at: 2/11/2013 (19:00)