The difference between 6D and 1100D is rather huge, other than size wise. The most important difference is the sensor size and the difference in image quality.
But now that we all know that ur current camera is 1100D, we have a tremendously important & practical question for you - do u have EF (NOT EF-S) lens? The 6D can only use EF Lens and no EF Lens. So if u dont have EF lens, you might have to factor in the cost to buy these lens. In general, EF lens are much more expensive.