I voted for 5 keepers.
In a deep and competative league like this one, I think teams who accumulate deep rosters should be rewarded. Allowing for 5 keepers, therefore, seems fair to me since it is rewarding the teams that have managed to accumulate more keeper level talent.
Also, there are 10 starting positions, so every team will still have to draft half of their starters and three more bench players. So there would still be a large turnover on players, (8 out of 13 per team) but the move to five keepers would just reward the teams who excelled at getting more keeper level players.
I know i'm just starting the league, but I feel that in any competative league, settings should be structured in such a way as to reward the better managers and not designed to assist the weaker managers in order to level the playing field.
5 keepers rewards the better managers so i'm in favor of it.