I think the modern style is to look at BB% and K% not BB/9 or K/9, and it makes a ton of sense.
If pitcher A K's 2 in an inning where he lets up 1 H and 1 BB and gets a no K out, he K's 2 every 5 batters, and walks one every 5 batters.
Play B might get 2 K's, but allow 2 hits and 1 BB and a non K out. He K's 2 every 6 batters.
They both have 2K/IP and 1 BB/IP, but pitcher A has a 40% K rate, while B has a 33% K rate. (Player B has a 16.5% BB rate, while A has a 20% BB rate.)
Then, there is K%- BB%!