The difference between 1440p and 4k comes to text clarity, and specific edges look cleaner. (As well as far objects appear more appropriate). If you can, I would certainly go for the 4k option (though it may decrease your FPS slightly). If you're looking for future-proof peripherals, then go for the 4k one. If your previous monitor was FHD (1080p), then any of the two you choose will have a huge difference. The difference between 1080p and 1440p is much more significant than the one between 1440p and 4k.