That makes my neck hurt just thinking about having to game with the monitor on top and looking up. Eye level is what they (and our necks) are designed for. In any event, I'd put the bigger monitor on top. Of course I'd buy an L-shaped desk and put the monitors in different places on the desk and move the chair between the monitors (it's what I have set up now for 1440p and 1080p between two different gaming PCs).