Unity or GameMaker: Studio?

Definitely Unity.

Unless there have been some huge expansions to the functionality of GameMaker since I last used it several years ago, I would say that Unity is the far more powerful tool.

Better for what?Model preview/beauty shots?Game development?Ease of use?Powerfull tools?Good documentation and support?Bunch of tutorials?
I'd go with UDK for all of the above.

For 2D: It's kind of a tie. Game Maker is easier to use and test concepts with (so I tend to go with that at first), but Unity also allows for more porting options. If you're only going with PC, Android and iOS, both can do that, but keep in mind that the mobile add-ons for both are quite expensive if you're just considering it as a hobby.

For 3D: Go with Unity. Game Maker's 3D capabilities are kind of mediocre unless you use third-party 3D plugins with it.

Unity

I would say unity, when i make 2d games, i prefer to do my game engine myself using library like lwjgl 2dslick or the xna framework and maybe using some physic engine like box2d or use unity also for this, I tried gamemaker long time ago, when i was in middle school and i didn't know programming, but , if it's not changed, i didn't like it so much. Unity it's good and more versatile, you can do both 3d and 2d(fake 2d with planes and so on but it's the same) games and you have obviously a lot of thing already done for you and a good and intuitive editor, also there are many tutorials, example projects and people online that can help you to learn how to use it.