S.M.
Love, love, love the Ergo! It is very comfortable for my baby. He was a bigger baby (9 lbs) so I did have trouble at first with the infant insert but now at 9 months it is easy to carry him. I recently saw a review on another carrier at www.deliciousbaby.com I think it was called FreeHand. It looked like less bulk and still a very comfortable carrier.
We used the Bjorn for my first baby and like the Ergo better as it doesn't hold the baby right by the crotch, it holds more on the bum.