I prefer 6. For a couple reasons. I can buy a case of 6 and use it for grouse and chukar. That way I don't have to have separate containers. Fill the bird vest with 6 and can grab from any pocket and be set. My chukar shots end up being longer distance, and I've also tagged grouse on the wing with 7 1/2 and had them keep flying with feathers floating in the breeze.
I think it's personal preference. Bigger bb's make bigger holes in the birds. Bigger holes, but less holes. If you shoot them on the fly that is. If you are gonna plink them from a tree, .22 to the head is best.