My previous dual monitor setups had been mix and match, using an older monitor with a newer, better one. For me it was visually confusing and less than fully effective. This spring I upgraded to a 27-inch 4K monitor, which I have really liked. A couple of months later a little windfall allowed me to get a second one just like it.
If you go with a two-monitor setup, I cannot express how much better it is that both monitors are exactly the same. The eye travels much easier from one to the other. There aren’t vertical jumps because the screen sizes are different. In fact, there is no distraction to the eyes because even the peripheral-vision look is the same.
Two good-size monitors vs one that is much bigger?
Sure there aspects of management and use where the one is simpler. But there are plenty of times where I’m really only using one monitor, and for me in such situations one 27" monitor is better than one 40" monitor. I can simply ignore or even turn off the second monitor if I want.
Of course all this depends on one’s personal preference. Hope you end up happy with whatever choice you make.