Whenever I have a choice, I always use Dart. The tools (analyzer, profiler, etc.) automatically make me twice as productive as when I use JS.
TS is a language where the primary goal is seamless JS interoperability. Presumably, it will always make design tradeoffs to maintain that.
Dart is a language where the primary goal is developer productivity. The team decided that it required a clean break from JS to achieve that. The tradeoff being that JS interoperability is harder.
So it's not a great surprise that TS wins in the JS interoperability front. Dart has made big improvements here recently and continue to do so and I expect the gap to continue to close but never quite go away.
Similarly, it's not a great surprise (to me at least) that Dart is a cleaner more productive language. As many have said ES6 has made big improvements over ES5 and coupled with TS has significantly closed the gap to Dart.
I see this trend continuing. With Dart maintaining an edge in language features + productivity and TS with JS interoperability ease, but the gaps in both aspects being small enough that it is a matter of personal choice and what best fits the project.