Well see that's the problem, the OP asked a meaningless question. He said: "should I use an eight inch or a nine inch knife," and got three pages of nerdy nonsense extrapolating the poignant differences between 8, 8.5, and 9 inch knives as if they were somehow founded in reality. The extra inch of length does not increase your edge's longevity, unless you are a idiot savant who manages to wear the entire edge at the exact same rate; it does not take any extra room to wield, because it's an inch, and if an inch of steel makes or breaks your ability to use a knife, you shouldn't use be using a knife (seriously guys, just choke up on it a bit); and lastly, it only might increase the weight--perhaps between two otherwise exact models--but I butter toast with a 300mm gyuto that weighs less than my 8" Wusthof, so there's clearly some grand generalizations being made here.
The only people who ask if they should use an eight inch or a nine inch knife are people who don't understand what they're looking for in the first place, and it's therefore the last thing those people should be worried about.
