Juliet. I think Juliet is more genuinely in love. That doesn't mean I think that Romeo is not in love. I think he is absolutely in love with Juliet. He is in love with her in the same way that he was in love with Rosalind in scene 1. The difference is that Juliet reciprocates his feelings instead of telling him that she would rather be a celibate nun.
Romeo is older than Juliet. She is 13, and the text doesn't explicitly say Romeo's age. From his age and his experience with Rosalind, I'm under the impression that Romeo is an experienced lover and Juliet is his next infatuation or conquest. He says differently, and he does marry her, but Romeo is also still an impetuous teenager. I don't exactly trust him.
Because Juliet is 13, I think this is the first time she has ever experienced love and falling in love. That's why I think she is more genuinely in love. She's falling for Romeo and falling hard. How hard? Well, she actively defies her parents' wishes to be with Romeo and marry him. She's also willing to leave a life of luxury and a set of parents who do genuinely love her. Romeo doesn't exactly appear to be that close with his family, and he's willing to throw away his name. That doesn't exactly scream commitment to a whole lot. With Juliet, there is no doubt that she is fully in love and committed to making it work with Romeo.