Zangoose is better really
Seviper has better mixed attacking stats, with dual base 100s, but is held back with a bad typing for an offensive Pokemon, low speed and low bulk. Seviper can be run with special or physical sets, due to equal stats so it gives it a little versatility, and it gets access to priority Sucker Punch as well as Switcheroo (in conjunction with specs or something) and Coil as a boosting move.
Zangoose is the superior physical attacker, with base 115 attack and a much better base 90 speed (Seviper is 65). Zangoose is a fearsome wallbreaker, with it's high attack. ability Toxic Boost (Attack increases by 50% if poisoned) and Swords Dance making it overall much more fearsome than Seviper. It also has access to Quick Attack, which is good for finishing and has decent coverage moves like Close Combat and Night Slash.