I just got my Babyhawk Oh Snap a few days ago and I LOVE IT!!!!!
My 18 mo old is a big guy at almost 30 lbs and he feels so light in the oh snap. I carry him mostly on my front but have tried a back carry and it works great. He always nurses in it and it's very convenient for nursing.
I chose the oh snap bc I liked how adjustable it is and how it allows for a very personalized fit, plus I have a BabyHawk mei tai that I love but need more support. It's 15-45 lbs so I should be able to use it until ds3 doesn't want to be worn anymore.
I have heard the O & A's are really great too. I like to get my carriers used bc they are softer and more broken in and cost a little less so you might check that out. I bought the oh snap new bc I couldn't find a used one listed on the site I usually buy from.
Have fun shopping!
Wife to dh, Mommy to ds1 12/2002, ds2 9/2005, and ds3 9/2008.