Always get the the best single card you can afford. SLI should only be used on extreme high resolutions/multi monitor setups where a single high end card is not enough, or when you already have a card and can buy a 2nd for cheaper. It makes no sense to start out with SLI on a single monitor unless it is above 1920x1200.
Undoubtedly, SLI / CF will have a lower cost per frame than the top end single card. Looking at the numbers, you can't even make a sensible argument to the contrary.
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
The 560 Ti - 900 Mhz got 862 fps in SLI for $420 at a cost of $0.49 per frame
The 580 got 616 fps as a single card for $500 at a cost of $0.73 per frame
Easy win for the SLI ..... 40% more performance and 580's cost per frame is 49% more
The 6870 got 701 fps inCF for $310 at a cost of $0.44 per frame
The 6970 got 526 fps as a single card for $340 at a cost of $0.65 per frame
Easy win for CF ..... 33% more performance and 580's cost per frame is 47% more
Using more modern cards ..... Hardware.Info uses AvP High 4AA, Batman AC Very High 4AA, BF3 Ultra High 4AA, Crysis 2 Ultra High Edge AA, Dirt 3 Ultra 4AA, Hard Reset Ultra 4AA, Metro 2033 Very High, Skyrim High 4AA and Shogun 2 Ultra 4AA
The 660 Ti got 932.3 fps inCF for $580 at a cost of $0.62 per frame
The 680 got 711.2 fps as a single card for $480 at a cost of $0.67 per frame
Microstutter in my experience is something somebody read about on a forum somewhere but didn't read about the part where it was tested on a pair of $120 GFX cards. I have never seen microstutter or screen tearing, etc on any screen (including the 120 Hz models) where the card cost was > $200. I built 14 SLI systems in the past year, 2 single card systems, one of which the owner has already upgraded
Well it depends on what your doing with the single/SLI GPU. I'm assuming gaming, which by my preference is to do a single GPU. It will cost you less (GPU(s), PSU, Mobo) and is cheaper to upgrade if you so desire down the road.
The best price/performance is usually between $150-$300, depending on budget and the other parts in the system. If you have a dual core CPU that is okay, I wouldn't put a $600 GPU with it. The same could be said about a $600 CPU with a $80 GPU, unless you don't care about graphics much.
It all depends on your price range, the system you're putting it into, whether you're into the simplicity of a single card system, care about future upgradeability, and what games you play.
I've had great experiences with SLI (8800GT, GTS 250) and single card systems (GTS 250, GTX 470, GTX 570) and I personally will be adding a second GTX 570 once I feel mine starts to become inadequate (still happy with its performance in Crysis 2 and BF3).
Jack also conveniently forgot to mention that the 660ti can have major problems on the minimum framerate because it has a crippled memory bus (he also forgot to mention things like how those numbers are for old drivers which is why you see nonsense like the 670 beating the 680)...
670 is by far the better option. If you're seeing the 7970 for $312, that's gonna be hard to beat though.