Posted: 25/02/2014 at 13:08
I'd go with the computer plotting, assuming you are pretty careful and use lots of points (especially if you plotted it in satellite mode rather than map mode).
I use a Garmin Forerunner 305, and dedicated runner's watches are generally considered to be much more accurate than phones. However, when I zoom in on a map which has been plotted from my watch, although it will generally have me plotted well, even on the correct side of the road, every so often it will have me zooming across the road, running through people's gardens and even houses. I'm sure I would remember if I had done that!
I consider any of them only a useful guide to get you in the ball-park figure. There are many instances of people running a particular pace based on their app or watch and missing their target in a race because of a small inaccuracy. The only way to be really sure is to enter a properly measured and certified race.