They're essentially the same, with very minor differences.

from DxO Mark:

At ISO 100 and ISO 200 The 40D has ever so slightly better dynamic range.

At ISO 800 and above the 1100D has ever so slightly better dynamic range.

At ISO 400 they're virtually indistinguishable.

Since both are fairly old, I'd go with whichever one has fewer hot pixels to correct for in post-processing. ¹

¹ As camera sensors age the number of "stuck" and "hot" pixels increase due to use as well as exposure to gamma rays. Even out of the factory one example of "identical" sensors will have more defective pixels than another. Those tested at the factory as defective are mapped out of the sensor's output in a profile stored in the camera's flash memory. There are various methods to "remap" the hot pixel map of a camera after it has left the factory, with varying degrees of success. They range from something as easy as leaving the lens cap on and covering the viewfinder while leaving the shutter open for more than 30 seconds using the "manual sensor cleaning" option in the camera's menu to sending it to a factory service facility. Note that as higher ISO settings are selected more pixels will show up as "hot" and the worst offenders will be brighter than at lower ISO settings. Warmer sensor temperatures will also affect hot pixel performance.