You're going to be choosing between 1440p/1080p IPS (60Hz) or 1080P TN (120Hz) for a high end monitor. Best thing to do would be to go to a computer store and check out the different types of monitors there. TN panels generally have sub-par colour contrasts and bad viewing angles, but offer great response times, with little or no input lag. IPS panels on the other hand offer very good colours with much better viewing angles, but (currently) do not offer 120Hz support (although some Korean IPS monitors do allow some increase over 60Hz) and a slightly longer input time. 1440p monitors also tax your hardware a bit more, but 1440p IPS monitors offer some of the best gaming experiences out there. However, screen tearing is a lot more susceptible on 60Hz monitors, and you'll likely want to use VSync as well, at a slight cost to input lag.
So, if you want very good colours, go 1440p/1080p IPS. If you want a very good fluid gaming experience, get a 120Hz monitor. People often argue that the human eye cannot detect frame rates above 60fps, but I believe there is quite a clear distinction between 60fps and 120fps, and it does feel a lot more fluid.