Mr Mxyzptlk vs Bat Mite
 

Who would win in a battle between these two other dimensional imps?

But not a battle against each other... a battle to see who could get their respective super hero to say, "uncle" first?

My money's on Mxyzptlk. He's far more annoying than Bat Mite, and therefore should be able to easily convince Superman into saying, "uncle" if only just to get rid of him.

Agreed. Batman is a bit more strong willed (read; stubborn) so Myx will win this one easy.