Of the two, based on the parameters of your question, the stronger case can be made that Juliet is the protagonist. Romeo follows the same pattern throughout the play: he acts rashly, with little or no foresight, and costs several characters their lives: Mercutio, Tybalt, Paris, and Lady Montague are firmly his fault (and, of course, himself, for being too stupid to realize that Juliet was still alive). He is a static character.
Juliet is more dynamic: at the beginning of the play, she is a bit boring, neutrally agreeable toward her parents, not really having an opinion of what they ask of her. They ask her if she will try to take an interest in Paris, and she gives vague, noncommital answers.
When she meets Romeo, however, she becomes much more assertive, though also deceptive: she begins lying to her parents and the nurse, something we had not previously seen her do. She speaks in asides and verbal irony.
She does act slightly more maturely than Romeo, even expressing regret during the balcony scene that she had so quickly given an oath of love to Romeo, But she is young, caught in the moment, and truly believes that she is in love. She has no idea that Romeo had wooed Rosaline.
Romeo, lacking any bit of introspection, could have realized that he wasn't truly in love with Juliet. His passion for her was only so great because she reciprocated. Literally seconds before he saw her, he was allegedly madly in love with Rosaline.
The majority of the action seems centered on Romeo. He persues Juliet, kills Tybalt, is banished, comes back to find Juliet dead, etc. If you were talking strictly protagonist by amount of story devoted to the character - I think you could argue Romeo.
However, I believe Juliet goes through the most change.
She starts in Act 1 with thoughts on marriage. At first
This shows innocent obedience based on dependence.
Juliet's character progression throughout the rest of the play is one of disobedience and independence.
- She meets Romeo. She disobediently falls for him - arranges a meeting time to marry him (behind her parents' backs).
- She marries him.
- When he kills Tybalt - she frets for a time, loses her nurses trust, then goes behind her nurse's back (to the friar) to work out a plan to make everything right.
- She fakes her own death - lying to everyone in her family.
- When she wakes up to find Romeo dead - she chooses death over anything else - this is the ultimate act that she has forsaken herself and her family for Romeo. She'd rather be dead than live without him.