Pick either, they are both excellent.
My preference would be the PC P&C.
b. silver efficiency.
Do not worry about 6 or 6+2 configuration. A 750w unit will not be good enough for sli cards like the GTX590 that requires two 8 pin connectors each.
But, unless you suspect a performance problem, or you think your psu is failing, why bother to change?
I think it will be more logical to upgrade in the future to one of the 28nm cards from Nvidia or amd due the end of the year, and not go crossfire. I doubt that the strongest will need more than 700w.