SCT Encourages the use of load type dynos like Mustang dynos, but I'd wager the vast majority of tunes by SCT dealers are done on Dynojets - we use a Mustang dyno at one place and a Dynojet at another. Personally, I prefer a Dynojet mainly because the dyno software is so much better and the printouts are much nicer.
The problem with either is usually asscoiated with turbo vehicles - you rarely can get full boost on a turbo car mainly because neither loads it quite like the street does - a mUstang dyno is better for this, but still not perfect - that's why you can tune it on the dyno, but you should check it on the street. As long as the MAF isn't close to pegging the dynotune will be 99% close to the street tune numbers, but not always. If the MAF on a turbo car is close to pegging on the dyno, it'll probably peg on the street. So if you have a turbo car, you should request the tuner do at least a 2nd gear romp on the street with the wideband and logger on the car.
Don