Helloo,well, answering your question, in my opinion it would be Stu because he is the most "experienced" in those things, but it could also be Billy because of his character and so on, although I recommend that you make Stu the dominant one since in most of the Fanfics that I have read, Stu was always the dominant one and although I also came across some fanfics in which Billy was the one on top, it didn't give you as much tension and emotion but it was interesting, I would recommend that you make Stu the dominant one but a little submissive and Billy the Passive one but with Attitude ^^
