Jesus, it's been so long since I've seen FLCL that I barely even ****ing remember what she can do (Other than molest small children).
What I do remember and assuming OBD wiki isn't pulling my leg means that they may be more or less similar speed-wise (Nui being powerscaled off of Ryuko and Satsuki), but in terms of raw power Nui has a huge advantage. Not to mention she's borderline immortal.
Nui should win, I think.
Originally posted by KingD19Well he could steal solar systems, but I don't recall Haruko ever "fighting" with Atomsk. When he awoke in the finale, she was pretty powerless to do anything to him.
Haruko fought with Atomsk, the most powerful Space Pirate in existence, and the guy could utilize N.O. fields so powerful he could summon entire galaxies with a thought.
He possessed Naota and she got pissed because she wanted Atomsk's power for herself. True considering how uber he is he clearly held back(also because Naoto seemed to have some control and mainly stayed on the defensive). But she wasn't completely overwhelmed and seemed to be capable of continuing to fight when he professed his love for her and Atomsk manifested as himself.
I'm not saying she could beat Nui because FLCL doesn't follow hard lines for power levels, but I do believe she could make it a fight.
That was only Naota partially manifesting Atmosk.
Neme, Haruko was able to slice machine gun bullets in half with a scalpel, casually dodge massed machine gun fire and in terms of strength she did this:
2.50
She's pretty dang strong. That thing flew out of orbit and I believe out of the solar system.