I have both cameras AND I had to do my choice for my next trip to Ethiopia.
For different reasons (but same discerning method) in both cases, I'd choose the FM3A. In my case, the "effective" risk I considered, it would be for personal safety. For a poor man, an F6 is a huge temptation to steal me, even exchanging with a DSLR to discover later it was "only" a film SLR. In your case - from what I read through your lines, but I might be wrong - your "effective" risk could be camera safety. While I'm sure the F6 could withstand the environment you go, a bump or an unexpected accident of whatever nature along your path might be much worse (money wise) on a F6 than on a FM3A, besides the FM3A is rugged enough as well for your trip.
I do see Nobbylon's point of view, yet I think we must consider the difference between "theoretical" and "effective" risk. None of us owning a Ferrari would ever go on a sand or gravel to test it right? Sand might enter the engine and gravel could simply scratch the expensive body.
Second, as someone said, it's lighter and in the end I'm not sure which kind of tricky situation you might encounter in a landscape you wouldn't be able to deal with. Rather, instead of Velvia, I'd consider the new Ektar and the new Portra 400 which have already showed a great scan capability by the right labs.
If you go through snowy places, a negative film might deal with snow better than a slide, in case you overexpose.
Of course, you might simply want to see slides through a projector or feel the thrill of a old and fashionable way to take pictures (but in this case F6 would be equally out of its place, while FM3A would be conversely more appropriate); in this case don't consider what I've just written.
Tripod? Yes. With a FM3A you might just get a good quality table tripod and save weight. This is especially true if you go there only with lower iso films, negatives or slides.
Last but not least, consider at least a short tele, from 85 to 105; an FM3A has plenty of options, from Nikon to Voigtlaender, to Tamron, Sigma, Tokina.... Despite your ideas, you might realize to want one when it's late.
I agree with filmfan - having a Leica M6 would be equally nice, but it would be a FM3A-like choice, not an F6-like.