In a vacuum, for a zone/triangle team, I would say Lewis for the rebounding. Looking at the rest of your team, I see that you need Lewis to play backup PF. The best way to accomplish that (IMHO) is if he is not also the starting SF. If you feel like you can live with a less than optimal PF backup at times then continue to start Lewis.
If you are the type who likes to switch between the 2-3 and the 3-2, I would go with Bradley because he is the stronger defender. If you have a (relatively) weaker defender at SF you are actually weakening the 3 part of the zone (be it 2-3 or 3-2) and negating some of the benefit of the particular set.