If your board can do x8 x8 SLI, you won't notice any difference. The PCIe 2.0 bandwidth at x8 is more than those cards will be able to saturate. I don't think any card is available yet that can saturate the x8 bandwidth of PCIe 2.0.
As far as choosing between the 2 cards, I would go for the newer 660's. Yes, they both do SLI, if that is what you are asking.
I knew that both could do SLI, I was just worried about bottle necking because the 660's are "3.0" cards, and although I knew that I could run one no problem in a 2.0 slot at x16, I was worried that it would bottle neck at x8 when I put in SLI.