To answer your title question, if you have the cash (and they aren't that bad price wise now) I'd go 4k over 1440p any day (you can always run a screen at *lower* than native but not the other way around).
I also know that there are some 4k Freesync monitors out now if you don't mind going AMD for your graphics card (the standard Fury card looks like a great buy for a 4k screen right now, neigh on as fast as the 980ti / Fury X and a good $100 cheaper). Also remember than you can run a G-sync screen on an AMD card, or a Freesync screen on an Nvidia card (obviously without dynamic refresh rates of course), so it isn't a total dealbreaker. The other thing to keep in mind is do you actually notice / get bothered by screen tearing or any of...