[herr_oberst]

You'll just have to ride a few to see what suits your back. If you think FS will work for you, then by all means sample the goods. It's the only way to know.

(All these upright bikes make my back hurt just looking at 'em.
Getting some of the weight off your ass and onto your hands arms shoulders makes a lot more sense to me, but thats just me... )

My only other input into your search is to look at cyclocross or dual purpose tires before you invest in a spare pair of wheels. You may see something out there that will work for trails and pavement. I had a nice compromise a few years ago that I really liked and you may find one too and save a bunch of cash.

Check BikeTiresDirect or some such web site to see what sort of rubber is out there.

Good luck in your search. Buying a new bike is a lot of fun! Enjoy the process.