Going through upgrades it's often a bit tedious to test site functionality such as creating and editing nodes. Can Simpletest perform this kind of functionality? Or do I need a solution like Selenium?
It depends of your Simpletest tests. Upgrade your site in a test environment (stagging ideally) and then run your Simpletest tests. If tests are ok you can be sure that everything covered by your tests is ok.
In general Behat is better than Simpletest for integration tests, while Simpletest is better for functional testing.