Since we're NOT talking about a boxer twin this don't really belong in GSpot, but we're comparing a "Beast" to a "Thumper" so it don't belong in either of those...
Back to the point. The F800 S and ST have been around so dependability shouldn't be an issue for most of the parts. If it uses an odd sized chain-carry a spare. But you gotta draw the line somewhere. You can't carry spare shocks without leaving something else behind.
Research the possible flaws of each bike and see what you might be likely to need.
Obviously it would be much more fun on the F800, so if I didn't discover any serious fualts reported with them that is what I would be riding on your trip.