Turkeys really aren't all that tough, you only need a handful of pellets in the head and neck to put one down. That being said, everyone wants to put as many in there as possible for the quickest kill. As far as what is the best choke, i'm sure a lot of people will recommend a lot of different ones, but chances are any stock full or super full choke will be more then enough at 30 yards. As you guessed though a lot of it depends on the load, as different ones will perform totally different in different gun/choke combinations. Not to worry though, most shotguns aren't nearly as picky as rifles when it comes to ammo, remember you are shooting a cloud of pellets, not a single bullet. So if i were you i'd start with the factory choke tube that you already have, and the cheapest box of turkey shells and try them, chances are they will shoot pretty good.