I have a 27" Dell S2716DG; G-Sync that I run at 2560x1440 ("2K") @ 144Hz from a GTX1060 w/6GB.
At that resolution & refresh rate I see an avg. of 70-90 fps for most games with zero stutter or screen tearing.
Got it on sale for $499.
Points to remember:
a. 4K is the future. More-and-more content will be developed/optimized for that resolution on BIG screens.
b. G-Sync vs. FreeSync; your GPU determines which you can use. nVidia for the former, AMD for the latter. No mix-n-match. You can drive ANY monitor from either type of card, but to get the special "sync" function that eliminates the need for v-sync within the application you need the right one for the type you have.
Size and resolution being equal, G-Sync monitors are more expensive as they require an additional chip (licensed from nVidia) but Freesync ones impose more processing overhead so might not be quite as speedy.
That solved my confusion as well! 4K is indeed the future, it is evolving pretty fast in my opinion.