Romeo’s feelings for Rosaline were not the same as his later feelings for Juliet. While Rosaline was just a passing crush, Juliet was true love.
Romeo does describe Romeo and Juliet in similar ways, in that he suggests that Rosaline is a peerless beauty.
One fairer than my love? The all-seeing sun
Ne'er saw her match since first the world begun. (Act 1, Scene 2, p. 21)
He is also interested in Rosaline for other reasons. He seems to care about her personality.
She'll not be hit(210)
With Cupid's arrow. She hath Dian’s wit,
And, in strong proof of chastity well arm'd,
From Love's weak childish bow she lives unharm'd. (Act 1, Scene 1, p. 17)
Yet Romeo seems more interested in the process of being in love than the woman herself. With Juliet, he seems to want Juliet. He hangs outside her window, just to watch her. Yet he goes nowhere near Rosaline. He just locks himself up and avoids people.