Wings can come down to personal preferences Ace, as some trim great for some people and not so well for others. The big thing to keep in mind when purchasing a wing is get the design for the style of diving you intend to do. Many companies used to offer combo wings, stating that they could be used for both single cylinder setups as well as small doubles. Those wings are little more than crap that will not be optimal for either application. If you plan on diving singles, get a singles wing; when you make the switch to doubles, get a second wing with the lift that you will require (I have yet to see anyone that really needed the 55 pounds of lift that most standard doubles wings offer, let alone the 70+ pounds of lift offered with super wings.) That said, I did used to dive DR Superwing with double 85's because the length of the cell did improve my trim. Manufacturers such as Oxycheq made note of this and began reshaping their designs a bit (I could never dive the old DR Classic wing as it was just too short and fat for my body composition.) Find someone that has a wing you would like to try and borrow it for a dive or two to narrow down your choices while seeing how different designs will trim differently for differing body types.

As for doubles and a bad back, I would highly recommend going with something like the lp 85. My back is so bad that they were originally waiting for me to give up diving so they could implant a morphine pump to provide a constant dosing to my spine (got crushed between two cars a few years back.) I had no problems managing the 85's as back mounted doubles, and love them even more as side mounts. I tried some 108's as back mounts and the extra exertion it required for me to get into the water negated all that extra gas I was carrying. I have dove with folks divng 104's and had them hit thirds (with me using 85's) before me more than once; for those with better sac rates I can usually come close simply by adding an 80 stage. The lp cylinders are easier to get full fills, and depending upon where you are and what your personal feelings are with regards to overfilling stell cylinders...(my 85's are usually dove as 120's )

As with most things in life, one size does not fit all. You just have to look at what is available and try it out to see how it works for you. You can make that super wing work, or you could get a wing that will be much more efficient for your intended purposes.