Lambda: Parallel batch and stream processing. Batch provides accuracy; stream provides speed. Merge in serving layer.
Kappa: Stream-only. Reprocess by replaying from message log. Simpler but requires replayable sources.
In practice: Most teams use neither purely. They combine patterns pragmatically.
Real-time dashboards? Stream recent data, batch historical.
ML features? Stream for online inference, batch for training.
Don't over-architect. Start simple. Add complexity only when latency requirements demand it.