Let's look at these men in a variety of ways:
1. Parental approval: Obviously Romeo is a Montague and therefore unacceptable. Paris, being a County is of noble enough birth to be an eligible suitor for Juliet.
2. Potential for true love: Paris and Juliet may have briefly met at the Capulet's feast, but Juliet was not feeling anything. Romeo, on the other hand, just had to look at her to capture her undivided attention. In terms of chemistry, Romeo's got it, Paris doesn't.
3. Ability to provide: Both guys are wealthy enough to provide Juliet with whatever she wants for the rest of her life.
4. Devotion: Both men are so devoted that they ultimately die trying to be with not even her live self, but her dead body.
5. Age: Both seem to be just a few years older than Juliet.
6. Ability to defend Juliet: In terms of strength, each of these men were willing to fight for her, but Romeo ultimately won.
7. Level of Romance: Romeo had a sweeter way with words than Paris. Every time Paris spoke to Juliet, he tried to be clever, but because her heart already belonged to Romeo, Paris' words dropped like bricks. Romeo, on the other hand, could say the most cheesy line and still win Juliet's favor. I mean come on Romeo, did love's wings really fly you to Juliet's window in Act II.ii?