I think Cayteelin makes it seem like a breezes because he/she is so passionate about it.
I think ultimately, if the decision becomes too difficult, go with what you love which is obviously Pysch. The ATAR score difference will depend on how you go in other subjects as well. If you think that Psych will be in your top 4 and revs won't be, obviously the extra study scores you gain in psych will have a greater impact on your ATAR. Honestly, it's often too hard to be expecting what will be in your top 4, but yes, it's possible that repeating psych will increase your ATAR.
I will repeat though, doing what you love is important and will help you to do well. That might be the deciding factor for you. Don't waste your education on things you don't find useful or interesting to you.