Go the triple on a Troll. Even empty they are heavy with racks and fenders etc. If you go 4 panniers there is a tendency to load them up because they handle weight real well, and who doesn't like a few luxury items. The lower the better for low gears, because if there's one thing that sucks with touring it's pushing a loaded touring bike up a big hill. So much easier to pedal, even if it's only at walking pace or slower, and the geometry of the Troll makes going slow pretty easy. To give an idea whats possible, with 9 speed you can run 40-30-20 and 11-40 with a RD- M591. Sunrace make a 10 speed 40T Cassette as well, not sure if the small gears is 10 or 11T. It takes some modding to the front spider and bolts to make the 20T fit, and tuning it so every gear works is an art form, but you end up with a gear range of 720%, from 13" up to 93" which covers most touring.