You may see a slight difference as with any difference between chipsets. You even see differences between the same chipset on different boards. However, for a single 5970 they will both run it at PCI-E 2.0 x16. However, if you would add a second on the P55 it will run them at x8 x8 or x16, x4 depending on the board. The x58 will run both at x16.
However, I don't see a need to run 2 off these cards unless you are gaming on a WALL of large LCD's