When do I use MiniTest::Unit::TestCase versus MiniTest::Spec?

I've been learning TDD/BDD using MiniTest. What I'm trying to figure out is what parts of my code should be tested with MiniTest::Unit::TestCase and which parts should be tested using MiniTest::Spec.

I understand the difference between unit testing and integration testing, what I can't seem to grasp from examples across the web is whether or not a TestCase and a Spec are both unit tests or if a TestCase is used for a unit test and a Spec used for integration testing?

Should I keep my quick unit tests in MiniTest::Unit::TestCase classes and longer integration testing, which more often describe features, in MiniTest::Spec expectations? Does it even matter, or is it a question of personal preference?