Which is Better: Flutter or React Native Expo?

Honestly, it really depends on what you’re more comfortable with. If you're familiar with one of them, that might be the better choice. Flutter tends to perform a bit better, but React Native has a wider ecosystem to pull from, which could help in some scenarios.

I haven't used Flutter much, but I tried React Native with Expo and ran into a problem with integrating NativeWind. It turned into a frustrating experience, and I ended up giving up after a whole day of trying to sort it out!

If you’re already good at Java Spring Boot, I'd suggest looking into more about how both options work for building multi-platform apps. Each has its merits, but starting with what you know might ease the learning curve.