Interesting question. I guess I'm a road-bike snob and know little about mtbs. So my opinion doesn't count for much. But if it was me planning what you're planning...

I build up the International. As I read (or mis-read) your note you are going to be doing mostly road-biking, and covering a long distance overall even if each day won't be so long. On a trip like that the most important feature would be energy loss per mile, or if you prefer, efficiency. And then comfort. Road bikes evolved to be efficient and comfortable. The balance between the two varies with the bike, of course. Maybe an mtb is good at both too, but I wouldn't know.

When you throw in the aesthetics of the International the decision becomes easy. Your aesthetics may be different.