the XR is much more enduro than the XT. the suspension is way better off road. its more powerful, too. if you can find one, you can put a 40 litre tank on. I've heard that Bert Harkins might have found some more recently.
weak points ... narrow seat makes road riding less fun unless you put small hoops on and motard it ;-) rear frame is not too clever so you'll be wanting to reinforce that. if you're going to load it up and let it rip off road, I'd suggest the heavier spring that K-Tech can supply. engine is bomb proof, but alloy kick-start isn't so you'd do well to find a steel one. eg early dominator.
one thing I really like about the XT is that it has a separate charging circuit for the ignition so you can keep going if the alternator packs up. not sure that's really enough to recommend it, though. XR, by my reckoning is a better bet off road, but harder on the butt on road.