I have a 8", 10" and harpoon. On my boat I carry all three.
I mostly just use the harpoon now, because it's easier.
But when using the gaffs I found I reached for the 8" more often, and found that the 8" gap was a bit large in most cases. What I'm trying to say is the 6" would probably be best in most cases, but I also think you should always be ready for a sea monster.