I have an R12RT. Love it!
Does everything that I need/want a bike to do.
I too have pondered over swapping to a K12GT. When the RT is in for service, I always ask for a KGT as a loan bike, and spend the day riding that. I'm always impressed by the power, acceleration, smoothness, etc, but when I get back on my RT, it's like putting on your most comfortable old shoes. If I bought a KGT, I'd need to change the screen, seats, bar position and footrests to get the 'RT' comfort level that I want.
I dream of a 'K1200RT'. If BMW made an RT rolling chassis with the K1200 motor in it, then that would be one hell of a good bike!
For me, since my dream bike ain't available, I'm sticking with the RT. If I was a rich man, I'd have both - the RT for two-up holidays, etc and the GT for solo hooning about on day rides.