Which 4k monitor should i get?

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I have the curved ultrawide G-sync PredatorX34 and it's a great experience. I have a single GTX 980Ti and an i5-4670 (3.4Ghz).

I ran Witcher3 (and both additions) with a minor tweak at 3440x1440 @ 95HZ and it was superb. Doubt I'd notice any difference even if 144HZ were available.

Just finished Rise of the Tomb Raider on similar settings which needed no tweaking although pre-rendered cut scenes remain in 16:9 and it, too was, beautiful.

Depending upon what games you play there may have to be some tweaking if the game is not optimised for 21:9 Ultrawide 3440x1440.

WSGF can help you see what games support that aspect and resolution and guide you on how to tweak the settings - if indeed they can be tweaked.
Since money is no issue what about a ultra widescreen (3440 x 1440)/curved @100hz? The Acer Predator X34 is quite a awesome monitor from the professional & customer reviews I've read.
http://pcpartpicker.com/product/WDcMnQ/acer-monitor-umcx1aa002

Between 4K/60 or 1440P/144 I would go with the 1440P/144 (or higher) since having the faster refresh rate was a massive improvement over 60 especially in FPS games like BF4.
Asus PG279Q ROG Swift 27.0" 165Hz Monitor
http://pcpartpicker.com/product/XvfmP6/asus-monitor-pg279q

 
How about a 4K TV that supports 1080p @ 120Hz? You won't need to worry about quality of the 1080p thanks to 4K upscaling. The ONLY drawback that I can find is the input lag (and G-Sync if you really need it). Other than those two, it can do what an X34 can do, even more.
 
I have the curved ultrawide G-sync PredatorX34 and it's a great experience. I have a single GTX 980Ti and an i5-4670 (3.4Ghz).

I ran Witcher3 (and both additions) with a minor tweak at 3440x1440 @ 95HZ and it was superb. Doubt I'd notice any difference even if 144HZ were available.

Just finished Rise of the Tomb Raider on similar settings which needed no tweaking although pre-rendered cut scenes remain in 16:9 and it, too was, beautiful.

Depending upon what games you play there may have to be some tweaking if the game is not optimised for 21:9 Ultrawide 3440x1440.

WSGF can help you see what games support that aspect and resolution and guide you on how to tweak the settings - if indeed they can be tweaked.
 
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