I use a 12 gauge O/U and haven't seen that kind of problem with either grouse or even woodcock. I'm typically using 7 1/2 shot size.

I don't think you would go wrong with either 20 or 12. Would depend more on what you like better. I think if you opt for a 20, you will probably be able to get a lighter gun. I don't have any problem schlepping my 12 around for a day, but to each his own.

Good luck.