Not sure on choke but the federal heavyweight 3" #7's do well in my son's Mossberg 500 bantam 20. I used to put Winchester supreme 3" #5's and they did well too.
I wouldn't use 5s in a 20 gauge. simply not enough pellets. Go with federal HW 3" 7's and a truglo SSX choke. If you cant find the feds, get the HEVI-13s in 7 shot