What is the difference and advantages of SDL and Unity3D?

They are very different things. Unity is a complete engine. It provides you with just about everything you need to make a 2D or 3D game.

SDL is a low-level library for audio, keyboard, mouse, and joystick access. It is more focused on 2D, but can be used as a starting point for an engine itself (using OpenGL). But it will give you more work because SDL is not an engine.

What you should use will depend on what you want to do, the size of your team, and so on. If you want to make a simple 2D indie game, SDL solves it. But if it's 3D, using SDL will require many lines of code until the first 3D model is on the screen. To make this 3D model walk then ...

In general, I think Unity is easier because it has many tools ready, but using SDL helps to understand more about the subject (OpenGL, for example), because you will have to configure and implement more things.

An important difference between them is the license. SDL uses zlib license, which is free to use.