Maybe the best is to use both techniques and if they come to the same result you're fine

If you use the wheel rotation measurement it also helps to do multiple rotations at once ... 10 is a nice number ... then use an accurate laser measurement device to pintpoint the total length and divide that by the amount of revolutions.

Another thing which I will try soon is to attach my GPS to my bike with duct tape and see what it reads. It's pretty accurate i was told.