Here's some interesting info from the Warmoth site about different nut materials and their corresponding sounds. I think my choice would be either brass or silver. I've had brass on a few basses - liked it. Guess it would depend on whether or not you'd want to add a little brightness to the P.
The info on neck woods under "Custom Options" just to the right of that is interesting too ... gives a brightness "meter" for a bunch of different neck/board woods. Pretty much every heading under "Custom Options" has some interesting info.