How much accuracy do you think that you need?

Wheel driven computers are as accurate as your set up. Any measurement error, however, will be repeated over the length of your ride.
GPS can give you some strange individual readings along the way if there is a lapse in the signal but, over the length of the course it can correct itself.
The thing I like most about GPS, however, is it's lack of set up. Move it from bike to bike without adjustment.

One other thing: During my road running days I volunteered to help manage several big races. One thing I noticed was how much all the front runners "cut the tangents" on every turn. If there are a lot of turns, that will make a significant difference in the actual distance traveled. Don't expect your numbers to match up exactly with anybody else regardless how carefully you calibrate your computer.