Which of these pieces is more learnable in 2 months?
Sibelius 3rd movement
Prokofiev 2 3rd movement
Depends on your learning style and technical and musical strengths and weaknesses. Ask your teacher if you have one.
When I face a decision like that I spend a day or so going through both pieces working at them a bit. Usually I notice quite soon which one seems to come into my fingers more naturally and which one presents more problems. It depends on your own personal strengths and weaknesses. Also if you have a musical preference it will make learning easyer.
I would say Prokofiev is easier (although not easy). Sibelius is very awkward technically.
Which do you like better? Prokofiev is tech less demanding, but if you really prefer Sibelius, working on it might be more rewarding.
Prokofiev. Whether or not it's doable in two months depends on your technical skills and how much time you have to practice.