I own both a 27" 1080p 60Hz screen and a 27" 1440p 60Hz Screen. I know 120hz is a lot better but this is comparing resolutions first. With modern day games, 1440p looks much much better, you can tell the difference between 1080p vs 1440p, very noticeable. 60Hz vs 120Hz is also noticeable, with 120hz much more sharp and crisp. 27" 1080p monitors are fine, even if you have had a smaller screen before it won't effect you much, you may even prefer the larger screen (like me).
From this, A 27" 1440p @ 60Hz screen is your best choice of monitor right now.