Having had both, here is my 2pworth,
I had a 1200GSA last year, thought i'd died and gone to heaven, this really is the best bike on the planet, ever built, ever. Did some trips on it, some very long, some short, absolutely faultless bike, end of. Fantastic engine/box/brakes/suspension/(what else is there?). 50 mpg to boot! Only slight niggle was the finish, No, sorry, being too fussy.......
Got rid last December, (company bike).
Then i bought an 1150 GSA...............
IMHO, first impressions were not good. Boy oh boy it's heavy!!!!!!, and, it's slower, not as good on fuel, doesnt handle quite as well as the 1200, Feck me it's heavy..............but........i love it! Did i mention the weight?
They sort of grow on you, hard to describe really, i have thought about this over the past few weeks, and come up with the reason....
The 1200 was/is very good at what it does, perfect in fact, in every way, but you know what the 1150's got that the 1200 doesn't?
Soul.......Now that might sound silly but i found in comparison, the 1200 was just maybe too 'clinical' for me. I want a motorcycle that growl's, shakes a bit, coughs now and again, has a few foibles, has a 'knack' too starting it etc, etc.
Would i go back too a 1200? in all honesty, probably yes, but i'd miss my 1150 though......
Smudger.