I haven't tried the Ergo or the Oh Snap. I love my Babyhawk, though. The last time I used it was this summer, around DD's fourth birthday, for about a half hour nature walk. (FWIW, she's a small 4YO--about 35 lbs now, and was less this summer.) I can't wait to start using it again with the new babe!
I've never tried an Oh Snap, but I've tried an Ergo and a BabyHawk MT. I'm not a super big fan of Ergos because I find the chest belt hard to do by myself when wearing on front. I personally love love love Olives and Applesauce for soft structured carriers. CatBird is another mei tai that I really like. I agree that trying them out in person would be ideal. Are you on thebabywearer.com? You could post in their forums to see if there's a mama near you who would be able to meet and try some things with you. Or post here in the tribes. Babywearers, IME, are always willing to bust out their stashes to help a mama choose. Good luck!