144Hz vs 1440p is pretty easy: What do you mostly do on your PC? Do you play FPS games almost exclusively and on an extremely high skill level? Then get a 144Hz monitor. Do you play other genres, do you multitask (i.e. your PC is not a pure gaming machine) or do you like watching movies on that screen? Get a 1440p monitor.
I've never seen a g-sync monitor with my own eyes, so I can't comment on that one. But keep in mind: You are buying into proprietary tech that will be 100% useless as soon as you switch to an AMD GPU and that may or may not be discontinued at any time. Definitely not worth it IMO.