Gary,
I know nothing about channel

For flathead, it is depend on size of bait....sometimes I use 8/0, and if my bait is small, the obviously 5/0 or 6/0. I do not use circle hooks much, but I do carry 8/0 circle hooks as well.
As Alex mentioned, the size/with of the mouth is different between flathead and channel.
But to me, either case, won't hurt to use bigger/badder bait to catch bigger/badder fish

....eliminate time bothered by small fish anyway
Sam