Between the two you list, the 800XC will be more fun for you. If you're doing more street and the occasional fire road, no worries. If you start loading it up with gear or two up, then you will want more CCs and comfort.
You might want to check out the Yamaha Super Tenere too. Much cheaper, much more reliable, shaft drive and bullet proof, but still a heavy bike like the 1200GS, so that is a detraction. The seat has a low and high setting. I'm 5'11" with a 31" inseam and on the low the Super Ten is quite comfortable. At 43k, I've yet to spend a dime on repairs.
If you're wanting to go cross country, the 800XC is a little lacking on road for highway speeds, but much more flickable and pleasant in the two lane twisties than any larger bike. The triple loves to rev and rewards you with wonderful response for it's size. It will certainly get you there. A tad more maintenance than the bigger bikes, but not a deal breaker. If you're a lite packer, no worries. If you like to bring it all, well, the larger bikes start to have an advantage. You won't be wringing the motor out for all it's worth in the wind at 75 mph. On the smaller 800, expect to run out of throttle now and then on the interstates. And be willing to pack a spare set of sprockets and a chain in the bottom of your pannier for long trips. Sometimes things don't last as long as they should. And it sucks to be waiting on sprockets for a couple of days.