The P stands for Polycarbonate (Plastic). The SA315 is only made in polycarbonate. But the SA314 (the predecessor to the SA315) was made in Aluminum (A), Black Aluminium ( B) and Polycarbonate (P). The SA314P was replaced with the SA315P, but the SA314A and SA314B are still made.
Exactly. Both use the same driver and magnet setup, however the 314 is a cast version and will probably take a tougher beating than the 315 will (not to say that the 315 is brittle or a piece of junk, but rather that ABS plastic is still less resilient than cast aluminum).