More than just being similar, all GTX 690's are exactly the same based on the reference design with identical components. Buy the cheapest one and/or the one with the best warranty and extras in the box.
It depends on your display. For 2560 X 1600 and triple displays, a Titan will give you better, stutter free performance, even though its average frame rate is lower. For a single display up to 1920 X 1200, the 690 would be faster and pretty much stutter free.
Keep in mind, if you want to SLI a couple 690's, you cannot. They are worthless as SLI while Titans can effectively go double or even triple.
By far the most cost effective route would simply be buying a second 670.
By far the most cost effective route would simply be buying a second 670.
This. You already have one, and two would be close to a 690 already.
Normally I would prefer a 690 over 670SLI, but the cost savings in your case make it an insane deal.
This. You already have one, and two would be close to a 690 already.
Normally I would prefer a 690 over 670SLI, but the cost savings in your case make it an insane deal.
That assumes that the OP has a sli capable motherboard and at least a 750w psu.
Since the op asked about heat too, sli may be an issue.
But, yes, from a performance point of view sli GTX670 is a strong option.
If heat, and noise are considerations, and $1000 or so is not an issue, I would look at the Titan.
It will need the least power, run cooler and quieter, and suffer no microstuttering issues.
Benchmark fps numbers of the Titan will not be as good as a GTX690, but the result may well be more pleasing.