To be honest, I'd be surprised if you could hear a difference that would be attributed solely to the fingerboard. The neck shaft lumber choice is far more significant. Lots of folks say the ebony fingerboard is bright, the rosewood one is darker, blah blah blah, but it's a comparatively (some would say negligibly) small part of the overall sound of your axe. Go with whatever rocks your world visually, and feels right under your fingers.
I happen to like the density and slickness of ebony, but on a fingerboard my fingers, ironically, rarely make contact with the lumber because I just don't press the strings down that hard. VIsually, I don't much care for purpleheart, even if it is a nice smooth piece of wood - so I'd go with ebony on that basis alone.
Good luck.