You do realize SLI (and all GPU configurations) have terrible price/performance ratios? You don't double your performance with 2 cards, more like a 40% increase. Plus, you need a more expensive motherboard, a more expensive PSU, more noise, etc.
Get a HD 7870, or better yet the PowerColor 7870 LE for less than $250 if you can find one in stock, and get better performance for less money. Or, spend all the money on a 7970, though that is probably overkill unless you game at a resolution great than 1920x1080.
Beyond that, Duke Nucome is right, and the cards are essentially the same, though MSI was recently caught tweaking a GPU to perform better, but at the cost of stability, so maybe avoid them.