its a toss up between Runkeeper and Endomondo - i use both depending on the mood/usage/training
Runkeeper is more accurate re: mileage but sometimes the gps doesnt want to work, so i switch to endomondo and the gps is always fine. However Endomondo is less accurate with mileage, you find you are not quite running as far as it says, but its not far out.
endomondo has live tracking so people can follow you on the website, you have to buy the pro version of runkeeper to do this at about £20 a year.
with the free runkeeper you can enter workouts, intervals, and get coaching, endomondo doesnt do this.
runkeeper is easier to plot routes on that you can save as your own routes (eg if you do parkrun), plus easier to enter a run afterwards on the website if you have forgotten your phone.
with both you can export runs from the website and enter on the other if you want to use both and keep track on both.
hope all that made sense!