There's no such thing as "better" but it is more accurately..."what is better for you?" or "what is better for this project?"
I use MySQL because it's simple and most of my database projects are fine on it. Most "nerds" that I know strongly prefer MariaDB for performance. I haven't personally tried it or felt the need to stray from MySQL. Do you have any experience in one or the other? What's making you look more at Maria?
MariaDB claims it has a much improved query optimizer and many other performance related improvements. Certain benchmarks show that MariaDB is radically faster than MySQL. Benchmarks don't however always directly translate to real life situations.