Its the pads. For street use, I can highly recommend Duralast Cmax Ceramic pads front and rear to all but eliminate the dust problem. They are also quiet and stop just as well as the OEM Brembo pads in day to day use. They do not last as long, I got 20k a set including a few track days on my GT500 but replacements are free when they wear out and easy 30 minute change. Also, rotor wear is nearly non-existant. I have them on my GT now and the wheels stay clean as long as the rest of the car.
Oh, I also use Armor All Wheel Protectant on the wheels when I installed the pads. Makes what dust does stick wash off with a simple rinse, no scrubbing.
Just one man's opinion that works for me.