I have heard a lot of good things about the Ergo. It's pricey, but it's supposed to be the easiest on the back. I did not purchase one because I had already invested in a lot of other carriers. For my son, I preferred a Hotsling. I used it in 4 different carry positions before he got to be too heavy and independent to use it. I loved it for nursing too. You just fold it, put it on, pop your little one in, and go.
I am definitely a fan of baby wearing. I have heard that the carriers where the baby's weight hangs on their groin can be hard on their groins and hip flexors. Ones I would stay away from, if you're not fully set on getting an Ergo, are ring slings (hard to adjust and ring usually seems to fall right on collar bone) and mei tei carriers with tails that tie (cut into waist and the tails hang everywhere). I don't know if this helps. But if I can save others from spending as much as I have on carriers, I'll put it out there.
Check out: babywearer.com . It's a great resource for researching slings.
Best wishes!