Originally Posted by
dabac
Opinions on mixing 8-/9-speed vary.
8- and 9-speed cassettes are equally wide.
So the sprockets are spaced tighter by some fraction of a mm on the 9-speed.
I don't think that you have that quite right. 7 and 8-speed cassette cog spacing are the ones within a fraction of a mm with one another. 8 and 9-speed varies by quite a bit and won't mix/match. Assuming OP has a 3 x 9 drivetrain now, if he wants to use an 8-speed shifter to save money, he'll need to buy an 8-speed cassette so I doubt he's going to save anything. I'd get the 9-speed shifter.