Read a comparison between the 1200/1500/1800.
The 1200 is a good touring bike for solo or two-up, but not for adding a trailer - harder on the bike because it relies more on a power band then torque.
The 1500 is good all round solo and two up touring and relies on torque for pulling additional items such as a trailer. It has been mentioned that the 1500 is the Cadillac of the GW, that you sit in it, not on it, but some of the cons are that it has the same HP as the 1200 at approximately 95, no ABS braking system (got to have something to protect us from ourselves), and no FI. My preference would be FI - FI is magic when it comes to bikes and is not that complicated to get into IMHO.
The 1800 is the consummate touring machine resulting from years of experience. It has been mentioned that you sit on an 1800, not in it like a 1500. It is more sporty than a 1500, and because of the greater HP and torque, is more suited to heavier loads and such. It is also a good canyon carver with an upgraded suspension.
All three are great bikes, and have pros and cons. Getting one that works and suits you is the key.
Out with the local GW chapter on Sunday and on initial start and acceleration, the 1800s had me (as did my friends Triumph RIII with his 2.3 litre. Of course even the 1800s would be hard pressed to accelerate with the RIII) - had to be heavier on the throttle sometimes during acceleration, but once rolling not a lot of difference.
The 1500 is not a Canyon carver, but the upgraded Progressive suspension makes it a very nice performer for what I do. I ride two-up a lot as well, and the cockpit ergonomics works extremely well with no modification - I'm 6'2".
Cheers