If you're building apps with machine learning libraries that are not available with pub.dev (ie. scikit learn, keras) on flutter, you are better off building with Kivy. For every other scenario, Flutter is miles ahead. Be it performance, UI components, customization, scope, type safety, speed, stability, native implementations of libraries like notifications, maps, payment systems, local storage etc. Not all python libraries are compatible with ARM that runs on most smartphones. Flutter now has support for certain tflite models that can run ML algorithms.
Our organization uses Flutter in the front end and a combination of Python and JS along with Firebase in our backend. I would recommend the same. Don't reinvent the wheel, use the tools best available and supported.
Just an FYI: don't do flutter tutorials online. Watch one and start making your apps, trust me you'll figure it out yourself and will be able to pick it up by doing.
Flutter is better it's easy to learn, build fast in both Android and iOS.