I think you're harder pressed to find the benefit sometimes of 120Hz compared to 1440p IPS or PLS.
1440p is something where there's always a benefit, you will notice it all the time. 120Hz may help sometimes in FPS games or Racing games as it will be smoother, but unless you're a competitive player or basically only play FPS games you'd be better off with IPS (that is just my opinion, it will differ from others). 120Hz is not going help you kill if you suck, nor is it going to make you top leader boards because you own one, yeah you may have a slight advantage over someone running 60Hz, but it won't be massively different unless you game at a high level where the slightest of advantages and a split second makes that much difference to if you kill or die.
Another thing to note is that you need more horsepower in your machine to run 120Hz in some games, which will cost more for the set-up.
Ask yourself, do you prefer eye candy in your games? Or do you want it to run smoother? If you like going into a game, looking around in awe at the graphics, then go 1440p, if you don't care how the game looks, you just want it to run as smooth as possible, then go 120Hz.