a_d_a_m

Just looking at the pictures of your Pack Rat, I'd say make that the lead candidate and just swap the front end out for your tour. It's meant for carrying weight, it's a bike you know, it's comfortable, has extra bottle cage bosses, and you're already used to riding it with extra weight up front.

Regarding the Pack Rat's brakes...well-adjusted rim brakes are fine 95% of the time, and discs also lose stopping power when they get wet. My little touring group is split 50-50 on disc vs. rim and we've all been stuck in the same rain storms...