If you have back problems, I would avoid ring slings. Although they look quite simple, it can be tricky to get them adjusted just right, and since they go over one shoulder rather than two it is easy to end up with the weight distribution lopsided, which then exacerbates back issues.
To be honest, I wouldn't generally suggest feeding in a sling from newborn. Breastfeeding is an art that takes practice - as is getting the knack of a sling just right. Trying to combine the two in the early days is a bit like trying to run before you can walk. It is much easier when they are a few months old and can feed semi-upright, and it is also much safer at this point....there is a (albeit small) risk of choking with a newborn feeding in a cradle position.
I have tried the normal bjorn but not the 'active', so although I'm not 100% sure how much difference the active one makes, I am sure that you could actually find something more supportive for your back than the Bjorn. It really depends on what sort of style of sling appeals to you.
If you like the idea of buckles, I'd go with something like the Manduca, which can be used from birth, but also will last a good deal longer than the bjorn, and can be used for back carries once the baby gets bigger too. Unlike some brands of buckle carrier, you can cross the straps over your back in a front carry, which I think distributes the weight better. The waistband will transfer a lot more of the weight to your waist and away from your upper back compared to the bjorn. It has an integral insert to use from newborn, and the back is adjustable in height.
If you prefer something simpler, that can still be used from newborn, have a look at the Connecta.
If you like the idea of a long wrap, I'd think about either a stretchy wrap (e.g. Moby, Kari-Me, Boba Wrap, Wrapsody Bali Stretch, etc) or a woven wrap. They seem a dauntingly large amount of fabric to start with, but because you tie them on they are very adjustable. Wovens have a bigger learning curve but can be used comfortably for longer than stretchy ones.
If you'd like something inbetween, have a look at Hoppediz's Hop Tye - it is made from the same fabric as a woven wrap, but has a shaped body that is very adjustable to make it nice and snuggly for a tiny baby.
www.slingguide.co.uk is a good place to start for looking at different carrier types. And if you can get to a sling meet or sling library (www.slingmeet.co.uk) you could try some things out to get more idea what might work for you.