I run what I've got on hand (12 ga) - be that 6s, 7.5s, 8s, or even 9s (if the shot size and weight are legal for that species).
What I've got on hand consists of whatever I find for less than $5/box during the year. Sometimes, only 6s are on sale; sometimes it's something else.
(Win/Rem/Fed only, most of the time. I won't touch Estate or similar 'bulk' brands anymore. Had issues....).
However, I generally only hunt Ruffed and Blue grouse. The smaller bodies don't tolerate meat damage very well. In the terrain I encounter them, a 5 yard shot is a long one. The average shot range is, literally, 5 feet! I generally just try to clip the spine, half way between the head and body. It makes for quick, flail-less kills, and easy identification by any wardens that want to inspect the carcass. (Blow up/off heads don't go over well...)
At that range, with that type of shot; even the wad is lethal. Shot choice, so long as it's legal, doesn't matter much.
---I just wanted to add:
The terrain I hunt in is
extremely dense woodland with marshy areas. There are no chances for shots on the wing; so it's a matter of spotting them before they flush, and getting close enough to not have to shoot through 12 shrubs and a dead tree. This presents the spine/neck shot opportunities on stationary birds.