Next to no difference.
Only the looks, which is down to personal preference, and the gearbox. The 1100 has a 5 speed, but the ratios are better while the action is poor. The 1150 has a 6 speed with a smoother change. First gear and sixth gear have been criticised for being too high/low.
Opinion seems to be that 1100's are better built.
I originally wanted an 1150, but ended up with an 1100. I'm more than happy with it.