Use cases v user stories

What to choose for defining app functionality? As an agile approach user stories remind me about putting uncertainty on someone else’s shoulders. Good if you can. But I often should define the initial structure for efficient further discussions. Slightly cut spec of subfunction level use cases (actor, goal in context, white box scope artefacts, action, precondition + trigger, outcomes, concerns) seems to work the best for my current user experience design process.