Viper matches the shape of an uncircumcised penis more closely than Cobra. Without a narrow spot behind the head, Viper may squeeze down less on the foreskin, improving longterm comfort for some uncut wearers. Viper may also have a more natural fit for the foreskin in the head area, with less extra space.
However, there are many uncut chastity fans who love wearing Cobra, and we expect many cut enthusiasts to be equally well served by Viper. Ultimately, the choice may come down to individual anatomy as well as stylistic preference!