I don't think it'll be that difficult to decide.
Outlaw can play some backup PF, allowing us to slide Aldridge to center, or run Fernandez and Roy at the swing positions. Plus, in a pinch you can even start Outlaw at PF if Aldridge gets injured. Webster really doesn't give you much flexibility in creating mismatches, and realistically he's only going to ever back up the SF position (unless both Roy and Rudy are injured).
On top of that, Webster has proven he disappears as a bench player, while Outlaw has proven to excel in the microwave role.
Lastly, I still think Outlaw has more upside than Webster. Better hops, better length, even a slightly better shooter. And when you look at where he started from to where he is now, Outlaw has clearly demonstrated the ability to improve. He started as a shot blocker with no other skills or intelligence. He transitioned to pure jump shooter. He then became a clutch jump shooter. He's now added a slashing game and at least a little bit of a handle, and the ability to consistently dunk in traffic. He's still a below-average defender, but for a bench guy with all those other skills, I think you live with that.
Webster just hasn't shown me that kind of growth. He's a better defender than when he started (and now better than Outlaw) but he still seems to be a spot up shooter.
I know Webster is two years younger, but Outlaw's logged 7800 career NBA minutes to Webster's 5000. It's not like they are complete apples and oranges.