My main is a 27" 1440p 144Hz w/ G-Sync. I swap out the secondary depending on mood and what I'm doing, etc. I've a range of other monitors including 1080 and 1440p UWs, 28" 4K and a couple of other high refresh panels but to be honest the secondary display doesn't get much use since I quit playing Eve Online aside from now and again when I'm doing something where colour accuracy is important or I just feel like watching a movie at my PC on something better than a TN monitor. Sometimes I put on a TV show on the secondary while working on something but of late I've tended just to watch it in a window on my main monitor side by side with other application(s).