Well, I have the LG monitor in the 27" variant, which I will be selling because I also have the 5K UltraFine. I was going to use the LG for the PC beast, as a gaming device, but the refresh rate and lack of gaming features pushed me to buy a QHD gaming monitor anyway. Plus, I went from 27" to 32" for more immersive experience.
Having said that, if you plan on working on the Mac (and even do decent, non-competitive gaming) then you should go with the 4K LG. You can adjust the scaling. The difference in image quality will be palatable. It took me a long time to get used to the reduced resolution on the PC rig. I still can't stand it for anything other than games. I have been spoiled by the 5K iMac, now the MBP and the UltraFine.
I find the crisp screen to leave me less tired and with fewer tension headaches at the end of a long day. That is my personal experience, of course. Another way to look it is this - you can always run the 4K monitor at QHD resolution, but you will never be able to run the QHD monitor at 4K, no matter what you do.
Edit: Also consider pixel density. QHD will look sharper on 24" or 27" monitor then it will on something as large as 32" because you have a lot more real estate to cover with the same number of pixels. This is typically offset by the viewing distance, but if you use the machine for productivity then you'll want to sit closer to the monitor and take advantage of the real estate, which is what I do. That's when higher pixel density will pay off, in my opinion.