Go with the 20ga. For insurance get a box of heavier than lead (ex. hevi-shot, wingmaster HD) 20ga turkey loads. They'll give you a little extra range, and a lot more knockdown power.

Match that with a good choke tube, and you'll be able to confidently bust'em as good as anyone...provided you can entice the bird into range.

Good luck and have fun.

FYI...Here's the dope from Oregon Dept .of Fish & Wildlife:

Legal weapons: Shotguns no larger than 10 gauge or smaller than 20 gauge; shot size no larger than No. 2 or smaller than No. 6; and recurve, long and compound bows may be used to hunt fall turkeys. Dogs may also be used during the fall season only.