While we might want to believe there is an ocean of difference between the love Romeo felt for Rosaline and the love he felt for Juliet, it is almost impossible to say that there is any difference at all.
Romeo is an impulsive adolescent who is in love with the idea of being in love. As the play opens, he pines away because Rosaline will not return his love. His father is so worried about him being up at night and asleep by day that he comes to Benvolio to find out what is going on. While Romeo agrees to go to the Capulet party, where Mercutio and Benvolio hope he will meet another girl, Romeo insists there could never, ever, ever possibly be another woman in the world for him.
That is, until he sets eyes on Juliet. At that point, Rosaline is completely forgotten, and Romeo is wholly in love with Juliet. The big difference between Rosaline and Juliet is not the quality of Romeo's love, but the quality of Juliet's response. While Rosaline rejected Romeo, Juliet falls in love with him.
The head-over-heels, all-or-nothing quality of Romeo's love for Juliet is very similar to what he expressed for Rosaline. It is difficult to know if it would have been any more substantial because the couple is only together forty-eight hours. The two hardly know each other, and we have no way of knowing if Romeo might not have fallen head over heels over another pretty girl a month or so after the marriage.