Tim,
I now have two Benelli M1S90's and have three barrels for them. I have an 18 1/2", a 24", and a 26" and switching the barrels off and on a single or both receivers has given me a bit more insight while patterning. For turkeys and predators I like the handling of the 24" barrel best. But the 26" is a very, very, close second and might, just maybe, possibly pattern best of all of them. For wingshooting the handling of the 26" is much better. There is a difference, slight but it is there, between the 18 1/2" and the other two. I did some test with a Remington 870 and a 20" and a 28" barrel - the longer one did markedly better. A Mossberg 500 with a 24" barrel after swapping to a 28" barrel the longer barrel shot enough better there was no real comparison. My test were done with the same choke and shell in the respective barrels. The shotgun target shooters aren't shooting stubby barrels either, so there is something to the theory.