This is interesting. On a healthy team the backup small-forward doesn't stand to get many minutes. But what happens when Leonard is out? Either Pop starts Belinelli and moves Green over (similar to what happened with Neal last season) or he promotes the third-string directly into the starting lineup and keeps his rotation intact (which he usually does). If the former happens, then Young is a better choice, since he's the better defender and the team could better handle large small-forwards with him rather than relying on Ginobili or Belinelli to spell Green. If it's the latter, then it might be important to have an offensive player next to Green, since both Danny and Splitter are currently lesser cogs in the starting offense. So Maggette would seem to make sense.
Boiled down, Young's probably the better player for a fringe-rotation spot, but Maggette is probably better as a spot-starter. Whatever role Pop wants his 15th man to play is probably decide who gets the last spot.