Both and it depends.
For general shooting, for me, it does not make a real difference. I've used an OVF for so long that I am comfortable with it.
In funky/difficult lighting, I find it easier to watch the EVF as I adjust the exposure/EC to correct for the lighting. That way I fix the shot BEFORE I press the shutter. Rather than shoot, chimp, adjust, repeat.
I set the EVF to display blown highlights, so I can see what is blown. The histogram is a partial picture. The histogram may show something blow, but I have no idea what, until I look at the blown highlight display. I don't care about a bright reflection off a trumpet, but I do care about the face.
For sports and fast action, so far, I prefer the OVF. But I have not used a high end, high $$$$$ mirrorless sports camera with an EVF, which may perform better than my EM1.
But just as critical as the viewfinder is the AF performance. As I understand it, in general, mirrorless cameras still lag behind dSLRs for sports AF performance. Again the high end, high $$$$$ mirrorless sports cameras may AF well, but not the prosumer level mirrorless cameras. As long as I need to get a $4,000 mirrorless, just to match the AF performance of a $1,000 dSLR, the mirrorless performance lags.
I am an old foggie from the film days, but I like the data display on the EVF. It is kinda like a 'heads up display.' I learned to ignore all that data and concentrate on the shot, so the extra data does not bother me, yet it is there when I want it. And I can press the 'info' button to clear the screen.