One of the main differences between NPD and HPD is that narcissist's need positive attention and histrionics will take anything.

Histrionics are saying pay attention to me! I exist! I'm a person.
And narcissists are saying you're paying attention to me because I am great.

That doesn't say that a histrionic doesn't have the ability to solicit positive attention like a narcissist and some don't. But they have the ability to obtain that positive attention. I know I do.

NPD have more structure I think and dedication. Histrionics are less rigid, free spirits.

But they're not completely different and neither is superior to the other.