xMLxDefault

With a 1080ti GSync will gain you absolutely NOTHING.  It is meant for situations when you can't maintain a reasonable framerate, which with a 1080ti you can get 120fps+ in any game on max settings at 1440p...
 
HOWEVER, the PG279Q is worth every penny more than the MG279Q.  Why?  One acronym: ULMB.
 
Running ULMB is the backlight flashing that clears frames after they are drawn, it means that you eliminate motion blur and everything looks crisp and clean.  It limits you to 120hz rather than 144/165, but frankly 120hz + ULMB looks and feels MUCH better than running a higher FPS.  The MG279Q doesn't have ULMB and that's why it's cheaper.
 
Don't skimp, get the PG279Q.  I had a Korean panel with a true overclock to 110hz and after going to the PG279Q it was like night and day playing online games, it's sooooo much better.  Trust me, best gaming monitor on the market hands down.

I got the mg279q and it is great, I do not see any jitters or stutters that g-sync supposedly fixes (but that could be me). As I will be getting a second identical monitor I opted for the cheaper version to save me some bucks in the future. I play games like BF1 & PUBG btw.
But for 200-300 extra bucks you get a more ROG themed style, a little thinner bezels and the G-sync/3D vision.  
 
Both are dope af

It's subjective. Personally, the PG279Q was definitely worth it just for the G-Sync.

I got the pg279q a few months ago and have no regrets. G-sync is great.

Adaptive refresh rates are amazing, and you really need to try it out. Personally I'd try for the PG348Q, but that's likely to be more expensive.

Well, according to this site "https://rog.asus.com/articles/gaming-graphics-cards/asus-announces-rog-strix-and-turbo-geforce-gtx-1080-ti/",  ASUS' cards will be out "from?" mid-March.