After listening to both albums, I have to say that
Secret is much better.
I really didn't like any of Ayu's singles this year, but I downloaded the album and immediately loved it to bits. (so much so that after one listen I went and bought it off YesAsia

).
On the other hand, I really liked all of Kuu's singles (not so much
JUICY or
Cherry Girl though ~ but that still leaves a lot of material that I enjoyed), so I pre-ordered
Black Cherry a while back.
Despite liking so many of the songs, there really seems to be something missing from the album. It's a little spastic, kind of all over the place. It doesn't seem to have a theme or vision. It's just a lot of music.
It's still enjoyable, but
Secret has a real flow, a real mood to it (even though it changes toward the end, it doesn't ruin it) that
Black Cherry can't match.
I still enjoy them both though.