I own a (Dean) 'niner and a fleet of road bikes, but I'm going to jump on the 'cross bandwagon here. Unless you're talking about some serious off-roading (I'll ride a lot of trails on 23c race tires...), having drop bars and a slightly more horizontal position is going to do wonders for your lower back. Nothing quite like having mountain geometry for a really long, not-totally-smooth (even FS doesn't get you away from the movements that will cause your back to hate you) ride...