I don't favor one over the other. And I dont' watch or listen to a lot of Korean music or drama unless my roommates offer them to me. My first drama was Full House with Rain (and I didn't know he's a singer then), and my first music was Seven because my roommate loves him. But I can say, I love both of their dancing moves. I love to dance, so watching both HOT Korean stars dance on stage is......LOL
Se7en is hot, sometimes I think he is only okay looking though, but he is a very good singer, love his voice, I am not sure about his acting since I have never seen him in a Korean drama before.
Rain, he's the cute, bubbly, adorable type and he's my favorite Korean actor. But he can be hot also like in A Love to Kill.. haha he had everything in that drama, hottness, cuteness, everything!! lol.. haha. I think his singing is okay I don't really like his songs only some I say her is a better actor than a singer no offense.
If I had to choose between them I say Rain! DUH.. lol! I'm a not big fan of Se7en I just love his songs and his voice.