Neither are good grammar. "I" and "me" are both okay. "I" is more poetic and hyper-formal, while "me" is more commonly used. But the rest of the sentence needs help. Consider these options: [Past Hypothetical tense] If she were stronger than me, she would have left it. [Present Hypothetical tense] If she were stronger than me, she would leave it. [Past Hypothetical Posh/Poetic tense] If she were stronger than I, she would have left it. [Present Hypothetical Posh/Poetic Contracted tense] If she were stronger than I, she'd leave it.

Hi there, Neither sentence is quite correct, but you're close! Here's the corrected version: "If she is stronger than me, she will leave it."

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