I agree with mustang - harness selection should be dependent upon boat design.
I have a DaKine harness that offers incredible comfort and support. Unfortunately, the end of the spreader bar attaches via a hook that takes my partners trapeze line out with me when I pass too close to it. While this isn't an issue on my Tornado (plenty of space between trapeze lines), when I sail the I14 (i.e. condensed cockpit) it is an issue.
Consequentially, for the I14, I use a Banks harness. It's fitted to me, is clean, nothing gets caught up in it. Unfortunately, it does not provide the same support, and gives me a wedgie if I overtighten the shoulder support.
For your back, I strongly recommend jogging, dead-lifting and squatting at the gym. Since I started doing the three of those regularly, my once bad back has felt invincible.