This is really difficult to say. But I would say both are really good. For short ones, I'd recommend highlights like cool kill streaks or funny parts with a lot of skips/cuts. And for the long ones, for really great story based games.
I love doing commentaries. It keeps the audience engaged instead of just recording gameplay. I'm sure there's people that prefer just gameplay, but from what I've experience, many like hearing and seeing.
At the moment I am playing PlayerUnknown's Battlegrounds, Dead by Daylight, Gears of War 4, Horizon: Zero Dawn, etc. I try to go back and forth so viewers don't get bored of the same constant game.