I have the choice of either the 20in or 28in accu-set barrel for my 8 shot mossberg 500. Will the 20in have the same pattern as the 28inch? I assume the main difference is the swing and extra length for aiming. I'm assuming i'll mainly use modified for the duck hunting and maybe some dove or bird hunting. I know I may have to mod it to limit rounds also.
Depending on choke, they should shoot the "same" as much as any two barrels are ever exactly the same. If you shoot at ducks, you will have to limit magazine capacity to two rounds. You may find you can hit ducks and doves with a 20" tube, but it would probably be a lot easier, and quieter, with a 28". Good luck.
Like the other poster stated, it's federal law that hunting for waterfowl requires 2+1 capacity max. Also you cannot use lead shot when hunting for ducks or geese. If you use your home defense gun, not only will you need another choke/barrel, but you'll have to put a plug in it while hunting. It would be SOO much simpler to have 2 different shotguns-1 for hd, the other for hunting. That way you wouldn't have to worry about changing it around all the time.
As long as the choke is the same, the gun will pattern the same-regardless of length.