You can't go wrong with a Mazda 6. You also have to remember that many of the other bodies out there..Ride, Yokomo.etc.. are also tuned by using different wings. This will affect the cars handling dramatically. In my opinion, and after extensive asphalt testing...the 6 is the best all around body. The 3.0, as someone posted earlier is better in my opinion on a big flowing track with high bite.
The bottom line is you just have to experiment. Driving styles differ greatly and this can also influence what body is best suited for you.
Butter