One main difference between Romeo and Juliet is that Romeo is rash and impetuous while Juliet remains more rational and practical.
We especially see Romeo's rash impetuousness when he rushes headlong into a relationship with Juliet. First Romeo risks his life by scaling the Capulet's garden wall just to see another glimpse of her. As Juliet herself phrases it, "... the place death, considering who thou art, / If any of my kinsmen find thee here" (II.ii.68-69).
Then he requests that they exchange vows of love, even though Juliet has already expressed that she thinks it is too soon to exchange vows. When Juliet asks Romeo, in her modest, embarrassed fashion, if he loves her, Romeo's response is to swear by the moon, though Juliet cuts him off, telling him not to swear his love for her at all. However, Romeo persists and asks for the satisfaction of "[t]h'exchange of thy love's faithful vow for mine" (133). The fact that Romeo jeopardizes his life and insists on promises of love even though they have only just met and know that their families will prove a significant obstacle for them shows us that Romeo certainly is far more rash and impetuous than Juliet, especially because she tells him not to make promises.
Not only is Juliet less irrational and less impetuous, she is the more practical of the two. When Romeo insists on swearing he loves her, her response is:
Well, do not swear. Although I joy in thee,
I have no joy of this contract to-night.
It is too rash, to unadvis'd, too sudden;
Too like the lightning, which doth cease to be
Ere one can say "it lightens." (122-26)
In other words, not only does she see making promises of love as rash and foolish, she sees it as being far too sudden. One reason why she rightly sees their vows of love as rash is because she knows very well that their families are enemies, and their families' hatred for each other will not only divide their love, but will also put their lives at risk if they are seen together. In addition, she rightly sees making promises of love as too sudden because she also rightly sees that hasty feelings of love are not necessarily true feelings of love, but rather more like infatuation. Feelings of infatuation are likely to disappear suddenly, just like lightning. Since Juliet rightly sees making promises of love as being too rash and too sudden, we see that Juliet certainly is the more rational of the two.