Most of the Mustang dynos use eddy current absorbers, whereas the very common/popular Dyno Jet dynos are inertia dynos. There is a big difference in the way that they work and measure power, and differences in their transient response, etc. Most of DynoJet's line of dynos now have an eddy current option though, so it really depends on which DynoJet or Mustang model you're comparing to.
This is like saying a Honda is faster than a Nissan, or vice versa. Well which Honda model is faster than which Nissan model...?