Hmm, I'll vote for Ayumi. Her music is still emotional and deep, whereas Namie's music is pretty much just the basic feel-good stuff and sexy lyrics, and a huge part to me personally in music are exactly the emotions and lyrics..
In terms of music, PAST < FUTURE surprised me as it grew on me after just few listens (at the few first I totally hated it lol) and I rarely really like the r'n'b-electro-dance music. But I have to agree with who said the arrangements on that album are not really original, but I enjoyed (most of) them nonetheless because they still managed to bring the Namie sound to them. But, I don't see myself listening to this album much in the future, maybe that's just like I said that I need something deeper.
NEXT LEVEL, then. I hated it at first too lol. But then one day it just clicked, and now I love it. What I like more about this album is that it has so much diversity and isn't just same elements recycled from song to song while still keeping the big picture. For me, NL is somehow so simple but so awesomely complicated at the same time: if you overall think about the songs they are very simple but if you really listen to them with your heart they get so deep and distinctive. Oh and the album makes me feel insanely good.
Also You were.../BALLAD is great imo.
Ayu's (imo) amazing tour also brings points to her, but it's hard to compare since Namie hasn't yet had done a tour since P<F... It's kinda hard to compare their "next levels" otherwise than just the albums because Namie's album was just out. :/