Even with my system 2x980 in sli, 4k can really push it down into some pretty low FPS depending on the game.
My experience with Aces High, and this is just mine, is that I prefer to play at 1080p than 1440p or 4k resolutions, due to the size of the targets(planes). At max settings the game now at 1080p is what I prefer. I also believe that 144hz/144fps is an advantage over 60hz/fps, but it's minor, and just my personal pref. I had a 970 (2 sli) before, and 4k gaming wasn't that great with them. I have an Acer 4k gsync and an Asus 1440p 144 hz gsync. Considering their high cost, and the experience, my previous 144hz 1080p monitors looked almost as good, like 99 percent IMO, and being 1/2 the cost or less than what I spent, is what I would get doing it over.
With your system, you're in the position to really push a 1080p 144hz monitor and use it to full potential, where as with a 4k 60hz monitor, you'll not be able to exploit it anywhere near to full potential, I certainly can't with a system similar to yours in CPU/MB/SSD performance, but a fair bit faster with GPU.
I find 27" 144hz Asus and Benq's on sale or good refurbs for 240$ Canadian which is under 200$ USD here all the time, and if I were in your spot, with your new system planning on mostly gaming, and mostly AH or other WW2 games, that's what I would buy, and save my 300$.
My shop I've used for 20 years just sold these refurbs out, but I know 2 guys that have them and swear by them, and they were around 250$ USD for a 27".
http://www.memoryexpress.com/Products/MX48351 . On a similar Asus monitor (144hz not 120hz, but close enough), my Aces High experience at 1080p, 144 fps in game constantly, at max detail, reflections at 2 or so, was excellent, and took full advantage of my 970 in game, and I'm certain it would be the same with yours.
Your first monitor, you could dial down the resolution for AH to 1080p, it's not like you are forced to play it in 4k/max, and desktop like I said looks great with that Asus monitor in the stores when I've checked it out, so it wouldn't be a horrible option, just not the best bang/$ IMO for Aces High.
