Inertial-type (Dynojet) dynos use mass (the drums) x accelleration = force (Hp/Torque)

Eddy-current (Mustang) dynos place resistance against the rotating wheels...and can alter the resistance at any given RPM. Vehicle weight, wind resistance, and so on, can be entered into the computer to mimic "real-world" conditions. In otherwords, you can mimic going up a hill while having the cruise control on. Great for tuning your truck that serves as a daily driver during the week, but pulls the camper trailer on the weekends.

Dynojet numbers are generally a bit higher than Mustang dynos because of the Hp/torque numbers are calculated somewhat differently and the vehicle will behave a bit differently while on the dynos. That said, you really shouldn't get hung up on the numbers. What a Mustang dyno offers is the ability (with proper tuning software, O2 sensors, etc) to really tune the engine under varying load, RPM, etc. which is very desireable for a daily driver or one that spends time on the roadcourse. A dynojet is a bit more difficult of a tool to use for tuning because if RPM is held steady, engine load drops off dramatically, which makes it a bit useless for tuning for load vs. RPM. Dynojets are a decent tool for testing new parts, engine combinations, etc. under WOT conditions