|"space is #1 concern in this build"|
Here's the dimensions of the 2 cases:
Corsair ITX = 13.8" x 10.9" x 11.4"
Cougar mATX = 7.09" x 14.57" x 16.73"
Other than the 5" height difference, I don't see any advantage in the Corsair's footprint. You may lose a few inches in depth but it's almost twice as wide. In fact, if you do the math, it's actually 47 sq.in. bigger. Plus there's still the issue of the PSU being mounted on the bottom rather than the top. But hey, it's your build, not mine.
The Problem of Bottom-Mounted Power
"I don't have space for 3X displays so going with one 1440"
And what will be the size of that one 1440 monitor? 25"? 27"? I have one of my PCs connected to a 42" HDTV at 1080p & I have no issues with display quality. And that's when web surfing, watching videos, gaming, etc.
"I read that 780 needs min 600W PSU"
Nothing wrong with getting a bigger PSU than "required", but just so you know, the GTX 780 itself requires less than 300W under full load (& possibly even less depending on the version).
Power Consumption Overview