i really don't know which one should be my priority...4k, gsync or 144hz
My opinion is this:
If you are a serious gamer and wants the best in terms of gaming experience (especially fps), have a high end GPU, go with 144hz. With g-sync if using Nvidia GPU.
If computer usage is split between not so serious gaming and non gaming related stuffs, go with 4K. For me, once I went 4K on 27", I really hate going back to lower PPI monitors. But that's just me. Of course, if you intend to game on 4K, probably will need minimum a GTX 1080 as well. However, the thing I like about 4K is its flexibility to scale down the resolution during gaming so that you can still game on higher settings, albeit on a non-native resolution (which may have some negative impact on how well the game looks, but it's a compromise with the flexibIlity you get).
The main reason I went 4K on a 27" is for the higher PPI, so texts are clearer. Of course, eye candy as well like what singupingu said. I also do some light photo processing, so a higher resolution screen is always better. I'm lucky because most of the programs I use do not have an issue with scaling. If you use a lot of legacy applications, then you might face some issues with the text or menu being too small.
