If you have a basically stock 350 270HP the 21 is what you want, some like the cleavers but a few of us have had bad experiences with them. For all around usage it is hard to beat teh stock round eared Merc props, if you want the best top end then some testing is in order as all boats even with the same power react differently. Chachis boat was stock the other two were modified and have no bearing for comparison to a stock setup.
Phil
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.