Mustang dynos are more accurate than most other dynos. They sometimes hurt feelings though lol.
A Mustang dyno is an Eddy current dyno, whereas a Dynojet and I believe dynocom are inertia dynos.
The Mustang dyno uses a load cell to actually measure torque where a dynojet uses a known inertia to calculate the numbers. It works almost like an accelerometer.
I wouldn't let the dyno type determine which shop I go to. In the end, the only thing that matters is real life performance measured by track times. I've heard of some of the shadier tuning shops with dynojets, attracting ignorant customers who have previously had tuning done with a Mustang dyno. The tuner says he made some tune changes, then prints out a dyno sheet showing higher peak horsepower, when in reality nothing was changed. Fortunately we have some pretty reputable tuners in our area, but I've heard of that happening in other parts of the country.