Kend,
I will put my 2 cents worth in too. If you look around at the most succesful Sporting Clays shooters, they will most likely be shooting an O/U in 12 guage along with several constrictions of screw in chokes. Many will also carry 2 and sometimes 3 different shell loadings with 7's up to 9's and many times different power levels for long shots vs close in. Sporting Clays is a real fun game, and can be quite humbling,but if you really enjoy it you might as well get a gun that will HELP you succeed in your effort. Clays certainly can be shot with a 20, I have even used my 28 and 410. Haven't done too well with the 410 because it obviously just dosen't have the reach or shot load to break the far ones. I have shot pretty respectable score with the 28 though. But when I am serious about it or at compitition, I always reach for the 12.
Good shooting