What will be better, 1 4k +1 G-sync monitor or 1 QHD?
First of all, forget Asus rog, it is overpriced and overrated. If you are to buy an expensive gaming monitor, buy the Acer XB270HU bprz. It is the only one that combines IPS panel, 144Hz refresh rate and G-Sync.
The cheaper alternative to the Acer above is the AOC below. It is only 1080p and TN, but it has G-Sync and its color presets and contrast are much better than the ROG.
On the 4K I would suggest to buy a true 4K one (4096x2160) and not the pseudo 4K that are being promoted right now (3840x2160). The only true 4K you can find is this LG: http://www.newegg.com/Product/Product.aspx?Item=N82E16824025007&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
If you need to go cheaper, I would suggest an IPS pseudo4K: http://www.digitalversus.com/lcd-monitor/benq-bl3201pt-p22555/test.html
For an even cheaper alternative, consider a TN pseudo 4K, the only one with G-Sync support too.
Monoprice is a fine monitor as far as color presets, input lag and stand quality are concerned, but it uses edge-to-edge glass which is extremely reflective and ruins the perceived black depth.
To add to the above, if you have an AMD GPU, replace the Acer XB270HU with Asus MG279Q. It uses freesync that works with new AMD cards. It is also cheaper, although G-Sync is a bit better for now, given that it works in a wider refresh range space. https://pcpartpicker.com/part/asus-monitor-mg279q