I think a lot of this argument depends on a few variables.
1.) Patrick O'Bryant - if POB starts slow and/or Doc has no trust in him and/or he just plain sucks, Glen Davis is this team's back up center. I don't care how short he is, he has the below the chest strength and size to push big men out of the way and just box them out for rebounds. If POB isn't doing it, BBD gets the backup center minutes.
2.) Foul trouble - in a perfect world none of our starters ever have to come out but if they do because of foul trouble, that will effect who comes out first. If Garnett, then I see Powe, if Perk then I see Davis. That's based on POB not doing anything. If POB is performing and getting soe playing time, then it comes down to 3 and 4.
3.) Matchups - there are just some players that Powe can't handle that Davis can and vice versa. That will be the first determining factor no matter what, as far as I am concerned.
4.) Davis' conditioning - if Davis comes into camp in better shape and, this is very important, maintains that conditioning through the year, he will get a load of minutes. He, unlike Powe, has a real offensive game and considering Doc's love of having two separate teams(yuck), there's not going to be a ton of offense on the florr with that second group. Davis in shape has the shooting, passing and dribbling skills to get his own shot off. Powe doesn't. If he is in shape Davis is playing minutes this year.
5.) The players ceilings - just how good can either player get? Have we seen the best that either player can be and if not, when they realize their full potential just who will be the better player. At the end of last year that player was Powe. But we may have seen the best that Powe can be. Only consistency would be expected now, maybe. But Big Baby hasn't scratched the surface as to just how good he could be. But will he ever get that good? Seriously good question that the answer to could be no. He may not be able to fight his eating demons or work hard enough.
So, all that said, either one could on any given night be the first PF off the bench. But if Baby is playing center, then the answer will be Powe.