They're nearly identical, and these benches would be run with double vs triple RAM too.
So no triple doesn't make much difference over dual channel.
The only real reason to get X58 (950) instead of P55 (870) is for the PCIe lanes. It lets you do a little more - either have SATA 3 if you have some crazy high end SSD or else add more GPUs, because two run at 16/16 while on P55 two run at 8/8. Still, a 5870 is only about 4-5% slower on 8 lanes vs 16 so unless you want to SLI/CF even higher end cards P55 is fine.