Performance
Given that the Define R6 and Define S2 share the same frame and an identical trio of fans, we'd expect cooling performance to be very similar, yet the S2 shows an early advantage. Slightly improved CPU cooling performance can most likely be attributed to the removal of the storage column - there's now a clearer path for airflow.
GPU cooling performance is a bit better, too, and that's because the Define R6 was tested with the optional vertical graphics mount. Our Radeon seems to prefer a more traditional orientation.
There's hardly anything in it when it comes to noise output. The Define S2, like the Define R6 before it, is very quiet when idle and does a decent enough job of keeping noise under wraps when the going gets tough.