ROW
I used to have a 1150GS, sold it for an 1150RT as the gearbox was about to chuck it and 15 months later have just bought a 12GSA. My thoughts are thus:
The 1150GS is an immensley capable all rounder. Compared with a Bandit, Fazer Thou or similar it will do everything you want of it and in the right hands trounce anything on back roads BUT it is seriosly quirky and a decent test ride is a MUST.
1150 v 1200 comparison
1150
very characterful, bigger, heavier, better weather protection, more 2nd hand add on parts, cheaper to buy.
1200
Lighter, faster, more torque, better handling but far less character.
I didn't like the 12GS and many times have thoguht about going back to the 1150GS from the RT but couldn't face buying all the bits I'd just sold. All the things I like about the 1150 and 1200 are combined in the 12GSA.
Conclusion
Apart from being a 100% personal decision, the difference is in how get on with the bike as a complete package. The suspension is amazing once you get used to it and the best bit that you can use every bit of the performance.
I once had a wee race with an R1 over the Kirkstone pass and got to the A66 some 40 mins before the R1 boy. Why? GS's are generally ridden by people who can ride it to within 90% of it's capability 90% of the time R1s (and similar) are generally ridden by people who can use it to 50%, 50% of the time.
They are damnably odd from the lack of dive (unless you're really trying) to the rocking at tickover to the indicators but you'll be with the programme on most of these already. Go for the test rides and then you can tell us