I've had all of them several times. Went for an 800 to a sertao, to a 1200 to an 800 and back to a 1200
Where to start
It completely depends on the riding you do, if you are a regular off roader I'd be reluctant to go to the 1200gs. It's big and heavy, it will do it but I think you'll have more fun on the 800 or the sertao. The sertao is obviously lighter than the 800 but feels more brittle in comparison, even off road. The 800 is a much better bike on road also, it's quicker, handles better and is almost just as economical.
If you don't off road much or at all then for me the 1200 is just the way to go. It's quicker but even travelling the engine just feels so relaxed. It's not as economical as the others but still reasonable and if you like the vague feel of the suspension as I do then it also feels better on the twistys. It's better 2 up and copes with this easily. Although the trend seems to be to downsize at the moment, I love the boxer engine and it's history too much to enjoy riding an alternative.
In terms of comfort. The 1200 is the winner hands down. The seat is not as bad as the others, the wind protection is better and the suspension is more plush. The 800 seat in my opinion is the worst and after a 2800 mile trip on it I thought I'd never walk again. The sertao is a little better but still requires something else to help it on longer trips. I got 70mpg on the sertao, 62 to 64 on the 800 and about 50 on the GSA and 54 on the non adventure 1200
This is a really brief summary, I could write pages on it, if you have any specific questions I can try to answer those for you.
