Learn both. I’m still learning Unity, but there are a ton of tutorials out there for it. I’m not familiar with GameMaker, but I’d say learn that too. Once you jump in and start learning the engines, you’ll figure out which one you like more.

I personally have never used game maker, and I think it is only for making 2D games. You can make either 2D or 3D games in unity, and it’s nice, although kind of buggy. But overall, unity is pretty good(I have only ever used personal edition). It gets a lot of updates as well. If you want to make super realistic games with super realistic lighting, etc, I have heard that unreal game engine is for you.

I use Unity, I have never used GameMaker but I do know people who use it. I’d say Unity and GameMaker are really very different game engines but they both have an equal amount of strengths and weaknesses.