I'd go with Torterra, but you won't be able to trade between games until after you get the first gym badge. Serperior is solid defensively but needs at least a turn or two of setup to actually be useful with any attacks, and with 5 weaknesses that can be a challenge. I suppose it can be a good staller, if you want to play that style, and after a few coils it can become a legit threat offensively, but I like both of the other starters in BW over Serperior, especially Samurott. Also Serperior has a much shallower movepool. Torterra is a much better choice in my opinion.