I chose the BECO carrier over the Ergo because it is more like a backback - the child has material in front as well as behind him/her. Once you undo the waist strap on the Ergo, the child comes out. With the BECO, the child remains in the carrier until you undo the straps and take him out. I found it easier to put my daughter on my back with the BECO because of this feature (you just swing them around gently onto your back). I tried an Ergo this summer and didn't like how the waist strap buckles on the side and found the whole carrier quite bulky. I guess you get used to what ever carrier you choose! They both have good reputations and both are excellent carriers!
I've used my BECO for my first child until I got too pregnant with my second child to wear it comfortably. I now use the same carrier with my son, who is about 20 pounds. I still, on rare occassion, put my 30 pound daughter on my back in my BECO and she's comfortable and I can carry her for an hour or so hiking/walking. My carrier is showing wear (fading...), but it's still quite comfy to wear and I think it will last until my son outgrows it.