A few differences. 11-87 has pressure compensating gas system to enable interchangable use of light-heavy loads. Not all the 11-87 barrels are so equipped however. My 20 inch rifle sighted barrel is not pressure compensating since it is assumed only heavy loads will be used. It still works fine with light loads though. 11-87 has stainless magazine tube. 11-87 has heavier duty extracter.
Before the 11-87 came along, 1100s could be had in 2 3/4 inch or 3 inch magnum versions. The 3 inch versions would usually work with heavier 2 3/4 loads but not reliable with light target loads. The 2 3/4 version could not handle 3 inch loads, obviously. 11-87 was Remingtons answer to a semi that would handle any load.
