How important is choosing a platform for a SaaS product?
I believe it is over-rated.
By this, what I mean is your dev team, your CTO, etc. will likely be most familiar with a certain platform. Or at least, most want to build on a platform.
All things being equal, optimize for the tech stack your team is comfortable with, or wants to build on. Not one that might be “better” but the team is uncomfortable with.
AWS, Heroku, Force, Google, Python, Java, whatever. Do what the team wants to do here.