Don't pay attention to price, or well don't take that into consideration. I'm not wanting to compare price I just want to game. Also I can push most games at 4k so performance or FPS of my machine is out of the equation as well.
I would prefer the 4k, as it is a higher resolution, without paying for the (in my opinion useless) 144Hz aspect. Of course if you did want 144Hz, that is a good choice as well.
Alternatively, you can buy a 21:9 1080p monitor like this;
The XG270HU will probably give the smoother experience because the high refresh rate and Freesync but the B326HK will probably have better color reproduction since it's an IPS panel. The XG270HU is a TN panel so the view angles aren't as good either but that doesn't really matter if you always sit directly in front of the monitor. I personally prefer high refresh rate monitors. Its the way to go if your system can handle 100 fps or more at the resolution you want to run. I've had six or seven different 60hz panels. I got rid of all them in favor of 2 high refresh rate IPS panels. I tried one in 1080p first to see if I could tell a difference between 60hz and 120hz. I sold the rest of my 60hz panels shortly after and got a 27" 2560 x 1440 120hz panel. The motion is much smoother.
I always prefer image quality over refresh rate. Smooth is nicebbut if it looks like junk there is no point to it.
Not saying the first one is junk but it is TN and the 2160p one is IPS.
I know you said ignore the price but there is always the 39" 2160p monitor that Wendell reviewed. And the other one he cannot import yet. I figure both of those are cheaper and either strike a perfect balance or out do the B326HK, so if I was in the market for one that would.be it.
I would probably consider this monitor over the XG270HU. The price is similar to the 4k and the panel type is the same. So, the chose would be high resolution versus high refresh rate and freesync for smoothness.
The ultra wide with 21:9 I've already got, i'm not to big on it myself. For work yeah its awesome but for gaming not so well. Don't get me wrong when games work on it, its great. But thats my point when games work on it. I have the LG 34UM95 but I can't game on it. Not enough support for it.