If most of your shots are at birds on the foot....410. If you think most of your shots are going to be at grouse on the wing...20ga. I use a single shot 12ga with trap reloads and its way over kill!!
What choke is in your 12ga? I am planning on using an 870 12ga. with a skeet choke and #8 shot.
I shot a 12ga for a while on upland birds and jumping woodies. I usually shoot a M or IC, depends on how long your shots are. For upland birds and jumping woodies, 7.5 M/IC works great and if you carry a few 6's, 4's, and BBB's with a modified you can take geese, rabbit, and beefy puddlers if you happen upon some.