A.P.
Are you set on a sling, or would any soft carrier work? I have the Beco Butterfly and I really like it. Baby/toddler can be in front facing you (good for sleeping) or on your back. What I liked about the butterfly as opposed to other soft carriers is that there is an inner panel between you and baby, so you can transfer from front to back without taking them out of the carrier.
Good luck!