Choke will vary by the loads you opt to shoot. If you use TSS #9, I got the best patterns with a Sumtoy .562, a Jebs .565 or Jebs .570. Not a lot of difference between them and I would not hesitate to use any of the three. This was in the 21" compact barrel such as yours. I would by all means try them in your longer barrels as well. I have a 26" barrel Franchi and use the same constriction in the chokes for it as the little 870. I got the best patterns, by a small margin using the .565 Jebs in the Remington and the .562 Sumtoy in the Franchi. These chokes shoot the magblends or #6 Longbeards pretty well also. I kept a .570 Jebs for the Remington and a .565 for the Franchi as backups....
There is a .555 Indian Creek on the 20 gauge choke for sale area. Many people on here have good luck with that combo on here....
