The Mustang Dyno (just a name brand, its not just mustang specific) factors your cars weight into the car. So if your stang weighs 3,300 lbs, it will add that weight, simulating like you would be driving around on the street. Some say this is more accurate.
The Dynojet is just a normal dyno, it doesnt add extra resistance because your car weighs more.
If i got any details wrong, somebody feel free to correct me.