The classic reasons to use ultrasonic are for male love handles, redo areas of lipo where there is scarring and tougher tissues, and male gynecomastia (breast development).
My experience is that the ultarsonic system goes through these tougher tissues and makes fat extraction easier and less traumatic. My experience is more limited with the SmartLIpo MPX system, which I also use, but my current belief is that it does not soften the tougher tissues as well as the ultrasonic system.
As already stated, choosing a surgeon who will skillfully use the best technology for your situation is far more important than the actual technology itself.
The makers of the equipment have gone a long way to promote their pricey systems on the internet, but your board-certified plastic surgeon is the one to look to for answers.