My buddy has a Viper and we ride every weekend, so I have some 1st hand knowledge of this. My Nytro is stock and his Viper has a few mods.
Drag race on the trail- Nytro totally crushes the viper, no contest.
In the powder- The viper does not seem to get on top of the snow like the nytro. Boondockin and climing you will be very pleased with a stock Nytro compaired to your Viper. Mine is only pulling 8150 rpm so I still have some clutching and gearing work to do for the 10,000 to 12,000 feet we ride at.(at 6500 feet it pulls 8900 rpm like it should) but even with the Nytro not quite dialed in, it is still no contest.
Here's 2 videos of the Nytro and Viper playin in the same field in Mancos, Colorado. The Viper goes good, but it's not as agile or as fast in the powder
as the Nyrto. The only thing the Nytro needs is a different track.
http://www.youtube.com/watch?v=QtC36dT0OvE
http://http://www.youtube.com/watch?v=Jue_yD6X2l4
Oh yeah, the Nytro only had 25 miles on it so it was not broken in yet.