Which subject is more difficult?

It really depends on your background, intellect, and such.

The General Theory of Relativity is a classical theory, and as such, is based in large part on concepts that make intuitive sense. Things start to get weird near extremes of speed or acceleration (aka. gravity), but that should be no surprise. But the mathematics are quite involved, and you have to take other people's word for what can or cannot be done to / with the math at the undergraduate level. If you like mathematics, just know that the Big Problems of GR are part of the introduction to higher level mathematics... and there are no closed form solutions in GR without "simplifications".

Quantum Mechanics asks you to "swallow camels" and "strain at gnats" day one. It describes a realm that few ever get a real feel for, but the "early" mathematics are not more involved than an undergraduate in a technical discipline would have anyway. The real difficulty lies in describing a behavior that you cannot experience, cannot train your "common sense" on, without some exacting physical setups. However exact its answers are, however well it describes fundamental chemistry, however our entire civilization rests on its findings, it is a "structural member" that is "ninety degrees to normal".

The real question is, will doing either of them well, net you a job you want? Difficulty arises from one's intent, not in the task to be done. And has very little to do with the subject itself. One cannot "avoid difficulty" in either topic by merely sitting in class... you need a goal other than a grade.

I'd agree. Some of the basic observations that go with GenRel seem very logical and sensible, and are based on much more predictable motions. Quantum Mechanics is pretty much the opposite, and (at the moment) reveals that at the smallest levels, there is an inherent amount of uncertainty and unpredictability to the way the universe works.

Generally, I'd say that quantum mechanics may be the more counter-intuitive subject.