last time we had a discussion in my school about your question...
as you say.......they both are right
But I'd rather use "If I were you..." because in that case it's impossible to be someone else.
Maybe you ever heard of "Conditionals".........
I'll give you some examples:
If you wore (past tense of "wear") that dress, you would look like your grandma.
If I had a cold like you, I would stay in bed.
If I were you, I wouldn't do that.
If I were you, I would tell him.
hope that helped
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