Depends on end use
I have an 870 12, and I can't complain about it. It's seen times in the turkey woods, squirrel, rabbit, etc, and has been to the clay house. Has never let me down.
My friend uses an 1100 for clay birds, and I've shot it. Honestly, I don't care for the twin bead, but other than that it was a nice gun.
Considering I already have an 870, I would probably buy an 1100. But consider the prices. If you're using this gun for clay birds or just occasional squirrel hunting go with an 870. Actually, to be bluntly honest, I really don't see a distinguishing difference, besides the fact of one being a semi and the other of being a pump. I've used a pump for quite some time and have never had an instance for a semi.
So, I guess I would go with the 870. Take it as you wish.