Hmmm... Hard to make a solid recommendation without knowing the age/weight of your baby, positions you like to wear him in, etc. But just going by what you said in your OP... In my experience a two shouldered carrier is much easier on the back than a single shoulder, especially once baby reaches that 20 lb or so mark. So if your main goal is to save your back I recommend something like a wrap or Asian-style baby carrier (structured like the Ergo, or unstructured like the Kozy).
Magical Mama, joyfully home educating my three wonders: FR (12/02), EG (05/05), DK (06/09)