Ok, well the first thing I will say against the 560ti SLI is the Memory, when I got SLI they performed well, but for example as soon as I turned the settings up in GTA4 I was soon out of Vram as it only uses the Memory from 1 card. This annoyed me.
As soon as I got the 660ti the lack of memory was gone, and the card performs very well, for example on the 3D Mark 11 benchmark my single 660ti scores slightly more than the 2x 560ti's, in Heaven benchmark it was not quite as impressive but still scored around 75% of the 2x 560ti's score.
In games the 660ti to me seems much smoother and copes far better at higher settings, no doubt a lot of this is due to the extra vram.
The 660ti runs far cooler than a single 560ti, not alone 2, and then of course is the power they use compared to the single 660ti.
If I had to choose, and I assume you are at that stage right now, I would sell your old card and opt for the 660ti, it's newer technology and is a very good card.
I am not sure if you are an Nvidia guy, but another card to take into consideration is the AMD HD7950, in the UK it is around the same price as the 660ti and I am led to believe it may perform slightly better. It's certainly worth reading a few reviews on both cards and looking at some benchmarks.
Hope this helps a little.