The bisexual idea would be more interesting than the two girls chasing the guy. Many teenage girls at that age already feel insecure and you don't want to indicate that life is just one big competition for guys and if you don't get the guy, you are a failure.
If you were to do the bisexual idea, you would have to convincingly portray the person in the middle's sexual attraction to both people. Sexuality is not as simple as heterosexual/homosexual- most people are on a sort of scale of bisexuality. If you could convey the complexity of sexuality to a teenage reader, that would be quite a feat.