The Hyper buggy is a great racing platform and has been a capable buggy since its inception. Anyone who saw Aigion's Hyper at the Worlds knows what it's capable of..
The Jammin' car isn't better or worse than the Hyper in any general manner, it just does a few things different is all. Either one of the cars will be more right for someone's driving style, just like anything.
Jay said in his interview with Car Action that he spent quite a bit of time testing and comparing his designs with the other cars on the market, and specifically noted the Kyosho car and its stability in rough sections of the track. I believe the Jammin' buggy excels on rougher surfaces and larger jumps, which seems to be the trend in tracks throughout much of the United States.