If you already ended school by dropping out or aging out, the only option is GED.
If you did not drop out, and have not yet aged out, you can get the diploma if you want to, or you can get the GED to end school early while being able to say that you graduated.
But, GED is basically the same as diploma. They both say that you graduated. The difference is, diploma only comes when you walk across the stage with your classmates, at the time that you were due to get it. Whereas, GED can be gotten at any point in life.