Which one should i use for better multiplatform code?
Just to confuse you a little more, I would add SFML to that list.
GLUT is an old library and I'm not so sure how well it scales today. Does it even support msaa? SDL supports more platforms, provides multiplatform audio, threading, networking, and controller support. It's more feature rich than GLUT. Both frameworks are rather old in design and IMO not that good anymore. If you're interested in building games, you might want to look at the various game engines out there today, like Unity. You can find and read more about game engines using the engine database here on devmaster.
I'm not interested in making actual games, but learning more about the game engine itself. Graphics programming was the topic of choice to start my learning tree. I'm a programmer, not a game developer. Game design / logic is boring