to be honest the primer you use depends largely on what the overall colour composition of the model will be like.
For example, if you were planning to do an elf where a predominant amount of the colours will be very light,,,its logical to use a white primer. Or if u were doing an imperial fist space marine (all yellow), this would be no easy task to do over black.
Also, something ive found, is that colours show up much better over white although it does recquire more work to paint over white than black, as when you work over black, if you miss out a recess whilst painting, it is black and usually not seen whilst if this is done over white, it becomes very obvious that you have missed an area.
When you first change from black to white primer, youll notice what a big difference it is, so i suppose, the best thing to do is just test out a model over white and see how it goes :bouncy:
chris