Compare the love that Romeo feels for Juliet to the love that he felt for Rosaline in Romeo and Juliet.
Romeo may have indeed been driven by two things for both ladies when all was said and done: physical appearance and attainability. Rosaline had beauty beyond measure (until Romeo met Juliet), yet she had this feature that made her unattainable, she wanted to be a nun. Thus, dating and sex were not an option for her. This usually makes a girl less attractive. I almost wonder if Romeo did not just like the idea that she was out of his reach.
Juliet was also beautiful, but at first Romeo did not know how unattainable she was. Upon learning it though, he makes effort all the more to know for sure that a future relationship was possible with her.
In terms of difference, many people judge Romeo's first love as infatuation. It was a crush, with no option to ever exercise feelings and determine if a love relationship was possible. Some might even say it was lustful because Romeo's words of adoration about Rosaline generally focused on her physical beauty and sexual appeal. Romeo's second love, to Juliet, is often judged as true love because it is a self-sacrificing love. Romeo is willing to give up on the feud and appeal to Tybalt as a relative. He mourns with Juliet over their separation. Willing to lose his own life to be with Juliet in heaven or wherever after life, many find Romeo's suicide romantic.
A final worthy note is Romeo's selfishness. Of both girls, Romeo wants what he wants and impulsively works to get it at all costs.