I have a Viper (as well as a Z) and trust me when I tell you that my friends 996 is invisible next to my Viper in a parking lot. Both are fast, both are fun, the Porsche is WAY more refined, but cost a lot more than a Viper (assuming you are looking for a Porsche with a comparable level of performance as a Viper - in which case it would have to be a TT Porsche). 488 CID and a blower makes for an exciting ride. There are a number of guys running 9's in Vipers w/ NOS (not my weapon of choice, though). Also very fast road course cars with a good driver. I might be a bit biased, though. :-)