Stick with the 22! Is the 18 really going to be that much lighter?
I use a one touch gold 22.5 for summer camping, and the q1200 for winter camping. the q1200 although smaller, weighs more, but being propane, it's very fast to cook on, which is good when it's freezing cold outside.
I leave my q1200 on the Weber cart all the time, the cart is mostly cheap plastic, and doesn't add much weight to the already heavy grill.
Have you considered going minimalist, and getting a camp fire cooking tripod? Then you just need the main grate from 22, and the tripod.