Here is an interesting article from Turbo Magazine about dyno's..
http://www.turbomagazine.com/tech/0306tur_dynodash/
I have been on a DynoJet 248 and Clayton Waterbreak AWD dyno..
The Dynojet 248 was always very close whenever I used it. They ahve a sunken 1000lbs roller in the floor. They have the most repeatable results time after time, seem to be the most fool proof too.