We own the Beco Butterfly II and the Beco Gemini. I tried the Ergo, but it didn't fit me right (it felt bulky), and also, you have to buy a separate infant insert for new babies. I don't know what the options are for the various carriers because I found it uncomfortable, so didn't really investigate.
If you are new to back-carrying babies, the Beco Butterfly II is awesome, because it has an interior sling that makes back-carrying so much easier. If you are an old-pro, though, you may find that the sling eventually kind of gets in the way a bit, so that is something to consider. However, like the Ergo, it does not allow a forward-facing option (I don't know where you heard that it does - check out http://frogmama.com, she does a wonderful job comparing the two Beco carriers for you). It also will not do a hip-carry.
My favorite carrier is definitely the Beco Gemini, though. The crotch-portion of the carrier folds in or out depending on the carry and the size of your baby, so newborns fit snugly without having to do the splits. You can wear the straps like a backpack or crossed for whichever support you prefer, and it does in and out-facing as well as back- and hip-carry (and I DO find myself using the hip-carry more than expected now that my kids are older!)
Both Beco Carriers will hold up to 30 or 35 pounds (I can't remember which one), and I can verify that, because during the Taste of Cincinnati last year, my husband and I carried both our infant and our two-year-old on our backs. 30 lb is a heavy backpack, but we were still able to manage quite well. Both carriers are more comfortable than the Baby Bjorn (I also have two of those - I love carriers) and my kids loved them (unlike the Moby, which appealed to only one of my kids).
Hope this helps!