There are a couple threads in the archives comparing the GT to Vipers, you might find them useful.
I've owned several Vipers, including a 2008 and still own a 1996. I want to say the Vipers are a lot more "crude" than the GT's but I also don't want to leave the impression I didn't like the Viper. The 2008 - to me - was a big disappointment even though it's generally the near epitome on the current Viper tree. If one compares the 08 Viper to a 96 GTS (or earlier) EVERYTHING about the 08 is better than the 96. Faster, handles better stops faster. Looks are more subjective (I prefer the earlier 96 GTS, but that's me).
Compared to the GT, the 08 I had - even with suspension upgrades (Moton's and Eibach's) - was a disappointment in most aspects. Any Viper has more off idle grunt than a GT (only considering stock for stock now) but after that the GT is far more refined in evry road-going aspects. Just driving down the road, the Viper seemed "bouncy", less glued to / stable on the road. Can't blame it all on big tires / tramlining because the GT is no slouch in the wide tire department.
I'll let someone with more track experience than me speak to that comparison.