Good decision on the 20 gauge, its all you need and with practice you can kill any bird that a 12 gauge will not get beat to death.
I have a Winchester Model 1300 pump shotgun, I recommend it because it has always been reliable and my favorite thing about it is the action is made to start the rearward motion for you and all you got to do is barely move it back and push it forward, this is to allow you to shoot it faster.
Mine was on sale brand new for $218 but it was marked down from $250.
I would recommend this gun to anyone, but you should really go to a shotgun dealer and ask to see different shotguns and see which one fits you best.