Alright, keeping in mind that I voted for Snivy, I have to say that Swampert is clearly the better Pokemon. One weakness, resistance to Stealth Rock, actual good stats, Water typing, Ground typing and actual good stats are but some of the things Swampert has over Serperior.
Really, the only thing Serperior has going for her are bucketloads coolness in looks and (the as of yet unreleased) Perversity/Antagonist/Contrarian/whatever-the-hell-it's-called-this-week mixed with Leaf Storm. The other Grass Starters outperform it in almost every fashion imaginable. Conversely, Swampert is considered to be by far the best of the Water Starters.
Snivy's still cooler tho.
Contrary is the English name of Antagonist/Devil's Advocate.
P.S : DEVIL'S ADVOCATE ? SERPERIOR IS THE DEVIL ? 0_0