Not knowing the exact model you are looking at, it is hard to make a detailed comparison. Overall, if you are looking at S2 Big Sky and Viper line-ups, here is how they stack up (I am talking about Viper, not Viper PST):
-S2 Big Sky has slightly better resolution
-Viper has slightly better contrast
-S2 Big Sky has longer eye relief
-Viper has less critical eye relief
-Both have good adjustments
-For the same configuration, S2 Big Sky will typically have a larger adjustment range
-S2 Big Sky line offers more variety (especially with higher magnifications) and both 3x and 4x erectors
-Vortex Viper has an exceptionally nice tweener: 2-7x32
-S2 Big Sky does not offer a tweener configuration, but has a nice low range variable: 1.25-5x20
-Vortex Viper 4-12x40, has somewhat significant (compared to other Viper scopes) tunnel vision
-Most S2 Big Sky scopes with top end magnification of 16x or more (and with 4x erectors) have 1/8 MOA clicks, which are too fine for my taste.
-All Viper scopes have 1/4 MOA clicks.