If you want one carrier that does it all, you have to wait a bit. The Beco Gemini
isn't out yet, but should be soon. (So "they" say.)
As far as sleeping goes, my guys have slept comfortably in ring slings, mei tais and SSCs, so again, I think it comes down to your baby and what position your DD will sleep in.
As far as which mei tai is best, it totally depends on the user and the baby. Some mamas love them and find them super comfy for front or back carries.
I had a babyhawk that I loved for front carries when DS1 was between 2-10 months, but after that it just became uncomfortable. I preferred the Ergo for back carries, the padded shoulder straps were SO much more comfy than the straps of the mei tai, so my Babyhawk didn't get very much use after that.
Is there a local babywearing chapter near you? It's a great way to try different carriers to see how they feel before you make a purchase. Our local chapter focuses on a specific carrier each month, but there are always a variety of carriers available at each meeting.