Wheelbase. The GT2 is the second evolution of the line and has a lengthened wheelbase. While this means little in practical terms (it is slightly better for overall handling), it does dictate what bodies each car can wear. Originally, all GT type on-roaders were based not just partially, but wholely on an existing 1/8th buggy, with little change but tires and body. In the case of the original Inferno GT, it was based on the Inferno buggy. Buggies have a wide fat stance, and as such, all the GT cars that came out before the GT2 were short nd fat compared to their real life counterparts, and subsequently had rather non-scale looking bodies. The GT2 and other cars of similar length wheelbase, are able to wear much more realistic looking bodies, due to the extended length.

For me, that matters, for you, who knows.... There may even be a long wheelbase chassis for the DM One that I don't know about (maybe I should ask RC Monster Mike if he feels like making one).