Kotlin would be the most direct comparison for sure.
There is much less boilerplate than Java and also just by the fact that it’s a relatively new language (~10 years old) that was allowed to grow without a large community attached to it for a long time but with an absolutely first class technical steering committee they actually had the chance to bring in a lot of really cool features without big messy community migrations.
Even those major migrations that have occurred (I.e introducing fully sound null safety a couple of years ago) were done in such a smooth way it gave me a huge amount of hope for its future.
I write mostly Java and Kotlin professionally but have lots of Dart side projects.
Dart has evolved a lot in the last few years and is now a wonderful language to work with, not only the language itself has lots of useful features (non-nullability by default, sealed classes and pattern matching, Isolates, List spreading, first-class await/async, stream generators) but it has high quality documentation/package-manager/IDE support. The fact you can hot swap code without fuss and benchmark it using the Dart Dev Tools is amazing (https://dart.dev/tools/dart-devtools).
Kotlin as a language has some features that are nicer (e.g. data classes - though Dart has just released the first part of its new macro system which may make Dart more powerful than Kotlin in this regard) and others that are worse (e.g. pattern matching), so it's a close call. In terms of tooling, Kotlin has wonderful IDE support as well, but lacks in terms of code documentation (its doc system was in "alpha" quality level for years, haven't checked lately but I wouldn't be surprised it's still beta), package management (it pigbacks on top of Maven, which is not so great IMO, it does the job but it's not as nice as Rust and Dart, have a look at pub.dev - it's so good) and its main build system (Gradle seems to be the most popular and everyone who has used it knows it's not that nice to use, though it's extremely powerful - but notice you can use Maven just fine if you're not doing multi-platform, at least).
If you want to do multi-platform, Dart is a clear winner because you'll have Flutter, which has an extremely polished UX and stable support for all platforms. Kotlin MP is still catching up, I have used it a lot recently and it's all just coming out of beta now (and a lot of peripherical stuff is unfinished). Kotlin has an advantage, however: you can write Kotlin common code and use the native UI system instead of going the Flutter way, which requires you to write the UI in Dart. So if that's what you prefer, Kotlin is the better option.
If you do backend web development, Kotlin is superior, I think, because it can use all stuff from the Java world (Spring, Jetty, all the instrumentation stuff, libs to do anything). But Dart is fine as well, specially if you want to keep one language for back/front ends. And Dart has better interop with JS and a well developed FFI for calling native code (which makes the lack of libraries less of a problem... it's not much of a problem anyway as Dart has A LOT of packages these days).
In the end, I think that which one is "better" depends a lot on pure personal preference, but regardless of which language you pick, you'll be alright, they're both great languages now.
