For gaming and if you want to play on 2 monitors (with games that allow that), than 2 same monitors. For productivity I prefer higher resolution and size for main monitor but I also have third, cheaper and smaller one just for monitoring computer parameters.
All together it depends on what you do with computer. For CAD work I use main monitor for entering data, secondary one to monitor results (real picture) and third one for calculator. Other jobs may require different setup.