Get to as many stores as possible and try different guns.
I saw earlier that Berreta, Browning,Ruger, CZ, Yildiz, etc., were mentioned.
Be aware that all of these guns fit differently from make to make.
Even within the brands they can fit differently , an older BL# Berreta for example is nowhere near the same gun as a 686 Onyx even though both may be field guns.
Check prices online as well, some stores have better deals than others at times.
The very best help you can get is to get with a buddy or meat someone at a club who knows shotguns and pick their brain , and get them to help you to figure out just what you need and want in a gun.
20's are generally built on a lighter frame than 12's so carry better, but with some heavier field loads they can recoil more than many people like.
I prefer light guns and light loads , personally.
Cat