On a practical building/finishing level, ash is open pored and needs grain filling, with alder you can generally get away without doing so. So you've got an extra hoop to jump through with the ash.
As for all the tonewood debate, I'm not saying it's wrong - I'm saying that I don't care. If you could put a number on it, if it influences the tone of an [i]electric[/i] bass more than 10% I would be very surprised. In my opinion pickups represent the vast majority of the sound produced, then string material/construction, then hardware, and finally everything else is filed under Misc and fights for the scraps.