Both players look good right now, but couldn't we say that about a lot of role players that had close to two full seasons playing with the Big Three and Rajon Rondo? What I fear is what we've seen from role players from the past that have netted big pay days looking good on contenders (Austin Croshere, Shandon Anderson, and Jason Caffey leap to mind): they aren't worth the money when they're asked to be the stars rather than the role players.
What does that mean? It means that if both players want sensible, short term deals, I say it's OK to retain both of them. However if they want 4-6 million a year escalating over the course of 4-5 years, I say good riddance to one of them and maybe even both of them. I don't think either is good enough to start on a contender and I certainly don't think that both of them are worth starter money. And I don't think either is the solution for when KG retires.
As for who to keep, I think we wait and see how the playoffs go. If both play well to keep Mikki Moore on the bench, we may want to consider signing both. In any case, I think who performs best in the playoffs may be the one to get the contract. I think it's a toss up at this point. Powe creates his own shot better, but Davis fits in with the starters better because he can create space with his shot. Ultimately, Davis may win out since come playoff time, the starters will play so many minutes.