The 11-87 comes with either a 3" or a 3 1/2" chamber while the 1100 has a 2 3/4" chamber. The 1100 is an older design - around 40 years - while the 11-87 is newer. Check out Remington's web site or better yet do some reading over at
Shotgun World. There's more info on the 1100 and 11-87 than you could read in a month.
Basically the 1100 is a great option if you don't need a waterfowl gun - especially for stuff like shooting trap/skeet/sporting clays. The 11-87 is the more modern design for handling the magnum loads used in waterfowling. But if you've got the $$ to buy premium ammo such as Remington's Hevi-shot, then the 1100 would even be fine for going after those big ole Canada geese.
The best thing about an 1100 is that a good used one can be had for around $300 and you'll never fear of not being able to find parts for it - even 50 years from now.
(No, I don't own one, but I may buy a used one for clays before long....an 1100, that is.)