To be honest I have shot about every choke combo out of my 870 youth model with the 21" barrel. The jellyhead just didn't perform as well as others. However, as long as you shoot lead, you will be limited to 30 yards regardless of what choke you have. Depending on what you plan to shoot, there are three chokes I would consider. The 21" barrel needs a little tighter choke due to its length.
Indian Creek .555
Truglo SSX .550
Sumtoy (you would need to call them to get the exact one)
All of these shoot Hevi shot and Federal HW shells very well. Good Luck!