It really depends on the scope of what you want to test.
If you want to test user interaction across controls for your application, OPA5 is a lot easier to do this. It will save you a lot of time manually retesting your application and nevertheless fits nicely with Test Driven Development.
If you are going to be building your own controls, Qunit will help you on these small unit tests for the getters, setters and other functions of your control.
Qunit has a list of do's and don'ts: QUnit Testing for SAPUI5 Controls - User Interface Add-On for SAP NetWeaver - SAP Library
Some limitations of OPA5: User Interface Add-On for SAP NetWeaver - SAP Library