I've been listening to SOUL'd OUT and RIP SLYME since their debut singles came out, but personally, I've always felt that J-pop fits them better than hip-hop or rap.
To me, their music seems to focus less on hard-hitting sounds, groove, or lyrical messages, and more on making rap and beats catchy, accessible, and easy for everyone to sing along to.
I'm not trying to impose my definitions on anyone. It's just that if I were organizing the CDs on my own shelf, I'd probably put them in the J-pop section.
I used to sing their songs with my friends at karaoke all the time when I was a student, and I still like them enough to go back and listen to them whenever I'm feeling nostalgic.