1100 is older model that was made primarily in the 2-3/4 inch chamber, aftermarket bbls are made for it if you want slug shooting or screw in chokes. They also had a 3 inch model but not nearly as popular, was only supposed to shoot 3" out of it..
11-87 is the newer design, many bbl options and many are 3" that can shoot 3 or 2-3/4.. Newest is the 1100 G3, it's an 1100 chambered in 3". All very nice.. Love my 11-87 slug gun and my 11-87 turkey gun but I tell ya I wouldn't trade my 1100's for nothing else comparable.. Just super nice guns..

What are you wanting it for?