I've been shooting a 20 since 1987. I find a 20 just as deadly on grouse and pheasant as any 12. It's a matter of the right load, choke and ability to hit the target. In fact, the only difference between a 12 and a 20 is the amount of shot it puts in the sky. Hitting the target is completely up to you.

I think you'll find that after shooting a 20 you'll wonder why you waited so long to switch to a 20. A 12 will feel cumbersome in your hands.
Just my personnel opinion.