I have a buddy that has an 870 SP, he had the Indian Creek Black Diamond strike choke in the 870. He wanted to shoot 3.5" but the gun/choke combo would only pattern a 3"shell. It really seemed like the 3.5" would basically blow the pattern apart causing a very poor close patterns.
I found him a JellyHead on here and traded the I.C. choke for the JellyHead. It turned the gun completely around. He shoots 3.5" shells with very good patterns now.
Its just a matter of finding the right choke/gun/shell combination. So I'd say start shopping for a choke tube, use the 3.5" shells and dont try to pattern both. If you wanna shoot 3.5" find a choke to shoot the bigger shell's and don't worry about the 3".
Good Luck!