if you've a bad back, myabe something 2-shouldered, as opposed to a sling?
A soft-structured carrier (ergo is one, there are many, many others) will last well into toddlerdom - you can simply swing them round to your back when they're a bit bigger. much comfier and convenient.
mei tais are also 2-shouldered adn offer good support, front & back, but you tie rather than clipping buckles. Actually not really any more complicated to use (some people find them a bit daunting at first, but it's all just a matter of trying and practicing a bit). If you can get along to a sling meet in your area to see a few things in action, that might be your best bet.

I prefer wraps - they're totally adjustable so will fit comfortably for anyone, can be used through till toddler years, front/side/back. very versitile. they do require a bit more learning and practice, however.

bjorns are not all that comfy - and most people wouldn't be able to carry beyond 3-6 months - they also don't really provide optimal positioning for baby's hip/spine development.