DO NOT BUY A BJORN! Okay, now that I have that out of my system...
Bjorns are terrible for both parents and babies. I had one and found that I could only wear DD until she got up to around 12#, then my back HURT really bad. The thing about the Bjorn-type carriers is they put all LO's weight on your back and shoulders. I found an Ergo to be the perfect carrier for my family because it puts all LO's weight on your hips, not back. An Ergo is a soft-structured carrier (SSC). There are many brands of SSC, including Beco and BabyHawk Oh Snap, which are preferable to Bjorn. They are more expensive, but you'll be able to use them for far longer than the Bjorn (I think the Ergo is rated up to 45#). You can also use them to carry LO on your front, back, or hip. Once LO gets bigger, it is far more comfortable to carry him/her on your back than on your front.
Bjorns are also not good for babies. They are known as "crotch danglers". This position can cause pinched nerves and arteries and creates pressure points in the spine. At best, this is an uncomfortable position for baby. At worst, it can cause problems with hip and spine development. You really want LO sitting back on a wide piece of fabric, resting on their bum, not their crotch. If you check out pics of the Ergo, Beco, or BabyHawk, you'll see the difference.
Good luck finding the right carrier for your family! The Ergo has just been wonderful for us and I'm looking forward to using it with my next LO, who is due any day now.