I'd advise definitely covering all usage scenarios with Selenium. And then parallelize. Overall unit tests should greatly outnumber UI/integration/acceptance t

I'd advise definitely covering all usage scenarios with Selenium. And then parallelize. Overall unit tests should greatly outnumber UI/integration/acceptance tests. And when you catch a bug with Selenium test, replicate it at the right place with a unit test. Look up "test pyramid" if you're perhaps not familiar with it.