Both guys have decent matchups but Buffalo's defense is poor IMO and I think Warner can do nicely. Like mr mark pointed out, Plax is gone as well so go Warner.
Start Manning. Hixon will be taking Plax's spot and he and Manning have a great relationship. If you saw him play in the preseason against Cleve you know what he can do. Manning will still have Toomer as well. Seahawks have a prob stopping the pass. I say Manning.
decided on warner. hes just more explosive. gigantes a more ball control type team. ithink both can put up 17+ points. i just see more potential out of warner.