Hello. I have $25,000 cash that I am debating if I should use to pay off part of my mortgage or invest it in index funds and let it grow. What would you recommend? I currently live in a townhouse in California with rising HOA and long term goal would be to hopefully get a house if the market drops. Thank you in advance. 
Welcome @Ivy!
I’d make sure you’re working your way through these priorities:
If you really are on Phase 6, then you are choosing from two great options! I’d probably make sure you’re maxing out any tax advantaged accounts (401k/IRA/etc) then choose what you feel best about. Here’s a discussion on the same topic: