Difference between Mantras and Buddha name invocation

[himalayanspirit]

What is the difference between chanting mantra and invoking Buddha's name?

For example, the normal Pure Land practice is to invoke Amitabha Buddha's name thus; "Namo Amitabha"
But Amitabha Buddha also has a mantra associated him ; "Om Ami Deva Hrih"

So what are the differences between the above two? Are the benefits of both different too? I am sure Buddha name invocation doesn't require any initiation or empowerment, but chanting mantra does?

Namo Amitabhaya