I love RTs and always have. Great commuter and holiday/ everything else bike. Bought my first in 1987 and had them ever since until last year. I had an 1100 RT for three years and an 1150 RT for four years after that. The 1150 is a great bike, very comfortable and the brakes are fabulous once you get used to them. As said before if you happened to buy an early one with servo brakes be very careful when trailing the rear brake in traffic or slow manoeuvering as the are very grabby. Later ones had a different bias to the rear and were less savage.
Of the two, I think the 1100 is probably the better RT. The five speed box is really all you need, the motor has loads of power and the bike handles extremely well and, without side cases, is not as wide as you might think. Also, despite what people will tell you, and the appearance, there is better weather protection on the 1150. The seat heights are different which permitted me to use the middle setting, with the screen just short of full extension, and ride with my visor up in all weathers. I could never find a setting on the 1150 that allowed me to do this and I was always wetter on the 1150 than on the 1100. Global warming possibly?
Another thing to consider is if you travel a lot in the dark. Does your commute keep you out in the evenings in winter. If it does, get the 1100. The lights on the 1150, although they look much sexier and have the spots built into the sides, are, to put it plainly, rubbish. On right hand bends in the dark, they are downright dangerous. There is an L-shaped cone of darkness, just where you want to see as you negotiate a right hand bend and there is no way to adjust the lights to eliminate this that I know of.
However, don't get me wrong. Would I recommend an 1150? Yes. Would I recommend an 1100 over an 1150? Probably. Is the R1200 GSA a GS with an RT fairing and better than both of them? Abso-friggin-lutely.