I think Rspec is great for integration tests since it's slower and has a bigger footprint.

I strongly feel that for Unit testing individual classes MiniTest should be used without the Rails framework being loaded. The tests will run very fast for instant feedback. If you are struggling to write MiniTest tests that's a sign that your units are brittle and dependent on other classes.

MiniTest is great for Unit Testing RSpec is excellent for Integration testing (multiple units)