I have owned both.The 1100 is gas operated,and the 11-87 has a "self cleaning" pressure compensating system.Basically they both operate from gas,but the 11-87 has a more advanced way of doing it,so it can shoot heavy loads,and light loads without changing anything.Back in the old days 1100's were sold chambered as 2 3/4" or 3" mag,which was used for 3" shells,and high brass 2 3/4" shells,it didn't work well with lighter loads.So they designed the cycling system in the 11-87 so that it could handle heavy or light loads with no changes.

1100 comes chambered in 2 3/4" now as far as I know.The 11-87 will shoot 2 3/4",and 3",andthe 11-87 super mag will take 2 3/4"...3"..or 3 1/2" shells.Either one would be fine upland,but I like my 11-87 more.If you only plan on shooting clays,birds,and 2 3/4" shellsgo with the 1100,but if you want to shoot turkeys,clays,geese,deer,anything that flies or runs from tree to tree,and larger shells.I'd go with the 11-87.It's pretty much an all around gun.