A couple of things to keep in mind about the GS800.
Firstly, as a new model you may well be doing the final R&D for BMW, finding out the hard way what they didn't get right. Secondly, you need to check dealer support for where you're going. One thing about BMW, if you can find communications and demonstrate you have the cash they will get the parts to you somehow, but you will pay.
The KLR is a known quantity, but the dealer support caveat still applies.
Which bike will tolerate crap fuel the best? Always worth a thought.
Does either bike use an odd sized part? Tyre sizes, for example. Chain sizes (the DR650 here in Australia comes with a 525 chain, when most replacements are 520 or 530. See what I mean?). Just have a good look at both and I think the answer will appear. Good luck with your travels.