Well Tim, your profile says you are interested in a 6.....
here's my opinion............
It can be argued that the -9, with the longer wing will get off the ground and climb a bit better than a similarly equipped 6.
if you do use an 0-320, they can be made to produce more power with the high compression pistons, but really, to get off the ground from 6,000', I think you'll want a constant speed prop, so you can wake up all 150 horses, and turn some decent rpm at take-off.
As many have and will say, you can't beat another 30 hp at that altitude, so the 0360 or even one of the new XP-400's will really make your day!!!
(Some smarter and better-lookin' experts will no doubt be along shortly to tell me I'm all wet!)