I've had steel and aluminium mtn and road bikes in the past. However, over the past 13 years I've been lucky enough to end up with three Merlins. My Merlin road bike was instantly more comfortable for my back than the Cannondale it replaced, but that could be size / geometry as much as anything else. Off road I don't think the material makes a huge difference to the ride. What I think you get with is a Ti frame is longevity.
I don't mean that it's indestructible – a big enough crash / rock will break anything – I mean that the Ti frames shrug off cosmetic damage really well. Forget about needing helicopter tape or cable rub damage. My old Merlin Taiga did a trip to the Rockies and four to the Alps where it spent a fair bit of time banging against the side of cable car gondolas and certainly didn't dent or even scratch. It's now 13 or 14 years old and has outlasted three pairs of forks. I've just sent it to Steve Potts in Point Reyes for a disc mount to be fitted.
I chose Steve rather than send it back to Merlin / American Bicycle Group as I had concerns about the robustness of some of their recent work. Which leads me on to the fact that not all Ti frames are created equal (just as not all steel and aluminium frames aren't equally well made). Cheap Ti is cheap for a reason, although having said that some expensive Ti has become hideously expensive in recent years for no good reason either. Steve Potts website shows some pictures of annonymous but big name Ti frames made elsewhere which have failed and been sent to him for repairs. Unbelievably the lazy b*st*rds don't always bother to weld the hidden bits of the seat tube / BB / down tube junction to each other.
So what's the conclusion? Maybe look for a second hand OLDER Merlin or Moots or Litespeed. They're still pricey and you don't get the benefit of the lifetime warranty as you're not the original owner. But I get the impression that now a days they're trying to build too light with too many fancy tube manipulations while still trying to contain costs. Result = failures.
Just my thoughts as a longterm multiple Ti frame owner (and a bit of inside knowledge from a friend in the trade).