I have both the Ergo and Packababy which though it looks like it has more ties than the Baby Back-tie, it doesn't actually, it has the same four straps. Acutally it only has two straps but they cross on the back of the carrier, rather having four sewn to each corner as does the BBT.
I like both the Ergo and the Packababy, but for my situation the Packababy now works better for us because I don't have a young baby only a larger toddler and the Ergo is just too short in the back for us now. The Packababy is very generously sized, this is because the designer wanted it to carry her 2 1/2 year old in it. But the Packababy can be used for much, much younger babies, even newborns which the Ergo can't.
So the differences:
The Ergo is more constructed it uses buckles, while I have found it to be very comfortable, a few people have issues with the shoulder straps digging in under their arms and making their hands go numb after a while. No one has complained about this issue in the BBT or the Packababy. The Packababy, like the BBT ties on, they do tie on slightly differently in that the BBT has the straps going over the baby's back and the Packababy goes under the baby's butt, unless you are using it with a tiny baby then you also tie the Packababy straps over the baby's back. IMO the Packababy (can't say with the BBT) gives more support than the Ergo and is easier to get on than the Ergo.
The Packababy ties are webbing and are a little stiffer, they do get softer with washing, the BBT's straps are fabric and tend to crease which doesn't help with the comfort, they need to be flatened out every so often or they will dig in.
So as I don't have the BBT, my vote would go for the Packababy over the Ergo. But the Packababy isn't nearly as pretty!
Have you seen the Sutemi Pack? http://www.sutemipack.com