I own an 800GS with 16k miles and I took a good test ride on the Triumph 800 XC last weekend.
The Triumph has a better engine and a better seat. The suspension seems a little better out of the box, too. I think it makes a better road bike than the F800. Also, the gearchange is a bit smoother than the BMW.
I didn't get to ride the Triumph off road, but the points that the other posters have made seem relevant. A little more vulnerable, a little more top heavy, and not as much low end torque. Also, first gear seemed slightly higher than the GS, and the GS is already too tall. None of these facts are deal breakers for the average rider.
Overall I thought the Triumph was an awesome bike and you can't go wrong with either one!