I'm a fan of both. It's hard to pick a favorite genre.
Right now, I'm loving K-Pop more because it's so fresh and fun.
Korean songs are so much more catchy.
You don't get anything super catchy like this in Japan:
http://www.dailymotion.com/video/x4a...babo-liv_music
Also, Koreans usually has better vocals too. They are certainly less nasally.