Tom, the 1150 is far more sensible. The 1200 was lightened (less strong) and more powerful (more highly stressed) so it is faster and probably more nimble if you are capable of telling the difference. After only about 800,000 miles of riding bikes I know I am not good enough to.
In two minds about selling mine now.....I had a Darmah SS900 that we had to sell, which I got the right sort of money for at the time (£3000). They are now fetching between £25,000 and £40,000 I heard for one minter. But then I think why worry.....live for the now.
The 2004 R1200GS has officially become a collectors item £9750
Check out this Australian gumtree add, people do live in their own little world
https://www.gumtree.com.au/s-ad/heidelberg-west/motorcycles/bmw-r1200-gs-2004-collector/1147729874