the beco and ergo would not be my first recommendation for a newborn (say under 4mo or so). i like to say they are too much carrier for a tiny baby, if that makes sense. however, when used properly with a newborn they are are still much better then a bjorn, which i don't think i would ever recommend.
i have three issues with the bjorn. first, they don't support the baby's legs in a spread-squat which is ideal for hip development. second, they don't provide proper back support for the baby's rounded spine, it forces them prematurely into an upright/straight back. last but not least, most parents find them uncomfortable, especially after 15lbs or so.
in my opinion the ergo and beco really shine with older infants and toddlers. the seat is designed to hold the baby in a spread-squat but is usually too wide for babies under 4mo. if someone could really only afford to get on carrier, they could definitly make them work with a newborn by putting the baby inside the carrier in a froggy position (upright with legs in, bum lower then knees). the beco butterfly actually has an internal harness to position newborns this way, but the older models do not.