Is masturbation or porn the problem?

Depends on your goals. Personally I think you should give up both for the 90 days to help your brain reset faster, harder but quicker and more effective and at the end of it maybe re-introduce healthy masturbation (once a week for maximum testosterone)

Both are a serious problem. Porn alters your reality and your mind, the way you think and the way you act. It poisons your ways and only leads you to more evil. Masturbation depletes you of your life force, your seed is made complete with various vitamins and nutrients. If it can create life itself imagine what it can do when it is not wasted in ANYWAY. 1 Corinthians 6:18-20 "Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body. Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your bodies."

Porn and masturbation are both a problem

Porn is the problem, masturbation is fine if done in a healthy way. Porn makes it into a toxic system in your brain. Just stop both for now.