I would tend to stay away from stainless, because of the expansion rate being a bit higher, and the heat transfer capacity being less than carbon steel.
Stressproof is nice to machine, but is pretty soft.
I would use C4140 HTSR (heat treated stress relieved) because it is somewhat harder. This means cutting it at a bit lower speed, but you only do it once
The taper should last a bit better with C4140, as will the keyways or splines, and any threads on it, etc.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)