The Viper has become something to really recon with lately ('97 and newer) and is far better than the NSX in many performance comparisons. Actually, I think it sweeps the NSX in every category lately except refinement. Its faster, quicker, costs less, turns more heads, is more easily recognized, and is climbing more enthusiasts lists faster than the Ferrari 550 captured the title of the Worlds Fastest Production car.
Rarely will you see both cars in the same garage. They seem to conform to two totally different party's and what those separate party's want out of a sports car. That would be either (1) Raw Power and unforgiving acceleration with that Miller Lite and the Fish and Chips. -or- (2) Pure refinement with not too much or too little power and a touch of elegance to go with that black Angus steak and red wine.
Personally, I love both and both are in my Top 5 favorites. However, the NSX is #1 and will remain so while the other 4 play musical chairs.
Opinions on this UBB are going to be baised but then again, this is an NSX forum and you wont see me hanging out at the Viper Pits.