Personally, I feel that if you find yourself thinking about the difficulty of being a player as a player or the difficulty of being a GM as a GM, you need to take a deep breath, take a step back, and consider if you really should be continuing to participate in that game. The question of whether an activity you do for fun is more or less hard really shouldn't be crossing your mind if things are going well, unless you're using "hard" to refer to "engaging challenge." I don't see that talking about how hard it is to be a player or GM really has constructive value. Now, talking about the "amount of work" or "amount of effort" or "amount of preparation," none of which are synonymous with "hardness," can be very constructive conversations, especially when some people in the conversation have never experienced one of the two sides firsthand.

In short, asking which is harder is kind of an odd question to me, since it isn't really something that should (if things are going well) even cross anyone's mind. And if things aren't going well... maybe address those individual problems rather than generalizing? And if you're just talking about in the abstract rather than basing it in real experiences, that's even weirder to me, since games aren't played in the abstract, and it's not like the conversation is engaging with some sort of game, like Theoretical Optimization conversations do (which I also find weird, but that's because I don't enjoy engaging in games in that particular way, so that's entirely on me).