Personally I'm not a big fan of SLI and Crossfire it's too much trouble especially now that you can find single cards that perform so well.
If you want to compare, you could put like one 670 from nvidia in the first system and put a 7950 or 7970 from AMD in the other one. The price and performance are similar for those cards so it would be a good match. Although the i5 CPU is a little better for gaming so I think it should give a slight advantage to the card that will go in that system. But both builds will be pretty high end anyways. If you can put a SSD in both for OS and applications you should have 2 killer machines.