A Dynojet can only take measurements at WOT due to its design.
A Mustang dyno is capable of holding a certain torque and allows you to tune at different loads and throttle settings. Much more robust than a Dynojet.
I don't know exactly what is meant by rougher on the numbers when comparing the two, but I guess it means that there is a bit more measurement error on a Mustang???
Of course, the Mustang is also a bit pricier than a Dynojet AFAIK, so the customer pays more to use it. For example, I can do 2 hours of tune with wideband A/F on my local Dynojet for $200 or so. The local Mustang dyno wants $150/hr plus another $75 to fit a wideband O2 before the runs.
Also, a 'dynamometer' is the name for the measurement device. Sort of like saying 'ruler' or 'thermometer'. Dynojet and Mustang are brand names of the devices. Those are the two common ones for what we want to do. (There are companies that manufacture chassis dynos for emissions tests, but it's not relevant here.)
