My main concern would be poor scaling on either platform with 2+ cards. You can't look at how well one or two games scale, gotta look at the big picture. You're overall satisfaction is what's important.
nVidia 2-card SLI is scaling very well right now, seems 80% or better on average across a broad range of games. That's where you want to operate, the best scaling possible across the broadest range possible.
With that said, I'd wait for GTX 280+. A 10% or greater improvement at the same or near same cost will make 2-card SLI a solid choice. Plenty fast and nothing wasted..