hey there,
given the events last week, the future pricing of bubble plans is still unclear. Pricing: the path forward from here - #27 by edd . We’re all waiting for bubble to release their revised pricing plans. I’d suggest you hold off on this decision for a week, if you can 
It also depends on the type of application you’re trying to build and what the database structure looks like. If it needs to be a real-time database, you’re better off using Firebase as an external DB and their suite of built-in Google APIs. In that case, you’ll probably be fine with a personal plan on Bubble + Firebase costs.