I have an ergo that a friend lent me and I really like it. You can get the infant insert and use it with a 7lb baby. I could be wrong, but I think the only Ergo that works as a back carrier is the performance. The Ergo I have only goes in the front. I was thinking of buying my own Ergo with a cute pattern!
All Ergos should be able to do front, back, and hip carries.
I love ours. DD is 20-22 lbs, and I can still wear her on the front without feeling any of her weight. I took her to the mall recently in our stroller and spent the whole time wishing for the Ergo. I often call it the best piece of baby gear we bought. For warmer climates (and like PP said, taller people), the Sport is the best option. I tried it on while PG. The material was thinner and if I remember correctly, there seemed to be less material in general.