Definitely some great points already made, but it's important to add that it isn't just the median difficulty of the questions that is important, but a) how you do compared to the average pool of test takers and b) whether the style of question is something that feels comfortable.
The GRE has a (deserved) reputation for feeling “easier” (its questions tend to be more straightforward), but for that reason is a bit less forgiving (if a test is easier for everyone, then every mistake puts you a bit farther behind everyone else). The GMAT, on the other hand, tests reasoning and critical thinking a bit more heavily, and in doing so ensures that almost everyone will make a few mistakes.