I was contemplating this fundamental debate over in my head last night, and came to the verdict of a 7 as the main de-animator and a 15 as a backup. With a bigger blade (21, 20, 9) I run the risk of getting the blade stuck while hacking at a skull, so what I need is stabbiness at close range. Were I proficient at loping heads off, seems like a katana would be the go-to. Katana BK-50??