How much if at all will the SX120 be better than the SX110?

The sensor size in the SX100 IS and SX120 IS is 1/2.5-inch. The sensor size in the SX110 IS is a larger 1/2.3-inch, but note that it is a 10.0 megapixel sensor (10.3 MP total) that has been cropped to 9.0 megapixels. This results in an image circle requirement that is only 2% greater than in the SX100 IS; uncropped, the image circle requirement would have been 8% greater. I assume that the cropping was necessary because the diameter of the image circle of the 6.0-60.0 mm lens used in all three models, which was originally designed for a 1/2.5-inch sensor, was not adequate for the full size of the larger 1/2.3-inch sensor.

One might speculate that there was no suitable 1/2.5-inch sensor available at the time of the SX110 IS development, but that one became available in time for the SX120 IS, although I have no information to support this speculation. Clearly, Canon's intent was to continue using the same lens used in the SX100 IS, rather than develop an all-new lens for the SX110 IS, which is quite understandable.