I have a 21" and a 28" barrel, also have a RSF and IC 555 choke for them. Which setup woud be better for 10" circle @ 40 yds. I will be shooting Hevi #6's when i get a chance. :turkey2:
Conventional wisdom says a longer bbl will give you better patterns at 40 yds.
However, our experience has been that each barrel and load performs different.
Get a handful of what you intend to shoot, take both barrels and both chokes to the range.
Shoot 3 - 5 rounds per bbl with each choke.
Then you have a good idea of what your barrels like best.
Try to do the testing on a day that will be representative of when you will be hunting. Yes, that is difficult, however, try to get close.
Good Luck
You will probably find that the IC .555 with pattern better in the 21", while the RSF may be better for the 28", being a .570. As was stated above, your best bet is to just try all combinations of them and see what you like. I carry a 28" 870 a few days each year and I love it. It patterns well with the RSF and Hevi 13 #7's. Some people find a 28" to be hard to carry sometimes through the woods, but it doesn't bother me much.