Easy question Casey. Larkin is dead. But now the he knows Kung Fu Chuck might give Casey a run for his money. Keep in mind Sarah nearly kissed Casey's ass in the Weenie shop early on in season one. Casey ran away out the back. Since then she got the better of him twice in Vs the Colonel. Of course they were both sneak attacks.
Casey, Obviously. Even if Bryce was still alive. It's Bryce going against a trained assasin, so the choice is really easy. Its not just because Casey's one of my favs. and I hate Bryce Larkin.