I have shot both 26" (came with gun) and the 23" out of my 870 SM. As far as pattern, apples to apples, the 26" threw a 308 at 40 and the 23" threw a 277 with Hevi 13 3.5" #7s.
I personally like the feel of the shorter barrel and that is the one I use. To me the shorter, slightly more maneuverable 23" out weighs the extra pellet count.
Hope this helps.