One of two big problems with modern bike wheels, at least for touring, is dish, and so I'd go with the XT to get the extra width.

The other big problem is build quality. You've got some good components in mind, so no worries there. But how the wheels are build will have more of an impact on how they survive than anything else. I haven't dealt with prowheelbuilder, so I can't speak to their qualifications. If you're going to get a machine built-wheel, find a good builder locally and ask him to touch it up for you. Read Jobst Brandt's book, "The Bicycle Wheel," and grill your builder. Does he know to stress-relieve the spokes? Will he tension the spokes high enough to be able to stress relieve them? How evenly does he try to balance spoke tension? Does he own and use a spoke tensiometer, or else can he persuade you he has enough experience to build wheels without one?

I've dealt with good builders in a hole-in-the-wall shop (literally! door was to an alley!), a major outdoor sporting chain, and a suburban bike shop. And I've seen otherwise good mechanics who can true a wheel, but don't do anything about adequate or even tension.

Buyer beware!