With the 1100 you can only shot one size shell in it. If its a 2 3/4" chamber, you have to shoot 2 3/4" shell (obviously). It go's the other way too, if its a 3" which there are some 3" 1100's out there, you have to shoot 3" shells in it even though the shorter 2 3/4" shells we fit, the gas system is setup for the 3" shell and the shorter shell won't work reliably, they'll jam up more then likely.
The 11-87 has a build in compensator so you can shoot both 2 3/4" and 3" shells interchangeably. If you get a 12 ga. with 3 1/2" chambers you can shoot 3" and 3 1/2" shells interchangeably. If you want to shoot 2 3/4" shells you have to change an "O-ring" in the gas system then you'll be all set. The fit and feel of the two are identical to one another!
