First of all, welcome to this forum and welcome to the wonderful (addictive) world of turkey hunting.
I have a number of readers who own Mossberg 935 shotguns and they all like them. The only complaint has been from those who did not thoroughly clean both the action, fire control unit and barrel. That barrel, while it may look clean when it's new, is not and needs to be "deep cleaned" at least one time, preferably prior to the first shot. The action and fire control unit may have too much oil/grease and should be cleaned at some point, again, prior to use is best.
If the barrel is not really cleaned, you'll never get the excellent patterns that gun is capable of shooting. Some of the best patterns I've seen have come from those guns, with barrels at least 26" long and using 3" shells with #6 shot.
My readers have had the best results from chokes with an exit diameter of .670. I prefer the SSX because it delivers great patterns and is very reasonably priced. It's also rated for all lead and "hard shot" loads, like Hevi-Shot, Xtended Range, Wingmaster HD, etc.
Lastly, while that gun is capable of chambering 3.5" shells, I'd recommend that you shoot the 3" shells. They cost less, pattern better and have much less recoil.
If I can be of any help, just let me know.
P.S. Here's an article on "deep cleaning" http://allaboutshooting.com/article_info.php?articles_id=242