Trail running is an entirely different story, and the GPS running watch must be very accurate or the terrain of the trail will confuse the GPS. The problem with rail running is that there are tons of tall trees, elevation changes, lots of curves, and even 360 degree turns on the paths. All this plays a major factor in the accuracy and can make the distance recorded meters to miles off the actual distance.
Garmin Forerunner watches are with no doubt the best and most accurate, with the 310XT models in first and the 610 model close behind. Some Garmin Forerunner watches can be adjusted to receive GPS signal every second, which will run the battery down faster, but will give a more accurate reading.
There is very little confusion for the GPS during road running, so accuracy is based the most accurate distances and speeds recorded. Garmin forerunners are again the winners on the most accurate GPS running watch for this category as well.
The 310XT is the most accurate, and the 610 is the second, but all Garmin watches are closely comparable for road running. Most other GPS running watches will work, but the 310XT delivers better results when it boils down to the actual distance that is travelled.