Input lag is stupid then I can't notice a difference from any other monitor I used. Input lag as in pressing buttons on the keyboard and having them appear on the screen right? I don't buy that test.
input lag is the delay between when you perform any interfacing action (keyboard/mouse) to when you finally see the result on the screen. Keyboard inputs are generally less impactful than mouse control of a fixed camera perspective as the control feels very rubber-bandy on a display with high input lag relative to a display with low input lag. If you haven't noticed it before, it is likely you've either been "lucky" and have had a low input lag monitor all this time without knowing it, or you've only ever had experience with high input lag monitors and have simply gotten used to it because you know of no other way. From what I've seen from my own experience as well as the experience of many friends/acquaintances, the effect of a high input lag monitor becomes far more apparent
after having experience with a low input lag monitor, and once you've experienced the low input lag option you don't want to go back (the same is generally true of moving from 60Hz to 120+Hz)
So basically its the 590D if you don't count the cost to push 4k pixels. And I never run my games at more than 60 FPS ever anyways so the 144Hz seems pointless. So its just gsync 1440p vs 60hz 4k.
of course you don't run your games more than 60fps, because there really isn't much point to do so if you have been limited by a 60Hz monitor
anyhoo, if you're toying with the idea of sporting enough GPU power to properly push 4K, you're probably going to have more than enough power to drive 1440p at a frame rate high enough beyond 60fps to appreciate the speed even before considering G-Sync...unless you're going to be pairing a bunch of high end GPUs alongside an AMD CPU...then it might make sense to stick with 60Hz. Although if that's the case I'd definitely consider the Acer XB280HK over the Samsung 590D because G-Sync on the Acer 4K will provide for a better experience, especially if you aren't going to go all out with GPU power. There's also the ASUS PB287Q, which is a 4K model similar to the 590D in specs/performance/price, only it has much less input lag, but again, $200 more for G-Sync could easily save you $300+ in extra GPU cost required to provide a similar if not inferior experience.
I still think the Swift is the best gaming monitor out of all these options, especially considering they're all TN.