I currently use RunKeeper and my wife uses Cardio Trainer. I like RunKeeper for the simple Start and Pause buttons. They are large enough to quickly press when I am basically out of breath at the end of a run.
As far as accuracy, I have mapped in our usual 4 mile run on google maps and my wife's Cardio trainer shows around 4.34 miles to Runkeepers 4.08. The mapped in route comes out to 4.05. So I believe that for our run, Runkeeper is a bit more accurate. This is the same everywhere we run too. If we use the high school track and jog 16 laps, mine will always be near the 4.0 miles while hers will show .24-.30 more mileage..
Just my observations and may be different in different areas with GPS and all..