What do you perceive is harder? Being a Player or a GM?

[PhoenixPhyre]

One other thing is the consequences of failure.

Consider a "holy trinity" MMO (ie healer/tank/dps). If a DPS is "bad", things still mostly continue, at least until you hit a (relatively rare) DPS check. But if the healer is bad or the tank sucks...everyone dies. So being a DPS is more forgiving and "easier" than being a healer or tank (usually, although there are exceptions).

Similarly, if the GM is crap or falls apart or is phoning it in, the game usually disintegrates or at least is very not fun for everyone. And replacing a GM usually (but not always) means starting a whole new game. A player who isn't firing on all cylinders or is phoning it in or is just there for the social part can often be left to drag in the water a bit or (worst case) replaced without doing more than marginally decreasing the fun of the table.

So the baseline stress/expectations for the GM are higher.

This is, of course, not a guarantee. Games and situations differ. YMMV.

And @gabj, as to your tangent question about "fun/rewarding"--for me being a DM 100%. A good game as DM supercharges me. It inspires me to worldbuild and gets me going like just about nothing else. A good game as a player is like, well, that was nice.

So when things are working well, being a DM is amazing. Bigger highs...lower lows.