Java lacks
huge low-level feature set C# comes with.
It is also generally less nice to work with - Maven and Gradle are a way bigger PITA than .NET CLI (which is similar to Cargo and Go CLI), NuGet and MSBuild. Base C# syntax lends itself to more streamlined expression of business logic (e.g. with pattern matching, tuples, records and their deconstruction).
In Java, there are odd issues and resulting method gymnastics caused by generics with type erasure, and many of its base containers don't unify nicely as the ones in C# do to IEnumerable<T> or Span<T>.
Writing highly concurrent + parallelized code is way more cumbersome (and generally less efficient) with the current rendition of virtual threads, completable futures or even upcoming structured concurrency API than doing so with .NET 'Task<T>'s and their composition.
ASP.NET Core is much faster and more focused than Spring Boot. EF Core is way more powerful and significantly terser to use than Hibernate, JPA or, to an extent, JOOQ.
You can also relatively easily ship fully self-contained and relatively compact (with trimming) applications, often as a single file. With additional effort, NativeAOT provides native compilation and smaller-than-Go binaries while having much wider support across ecosystem than GraalVM Native Image within JVM space (e.g. it's one command away to get a gRPC-based ASP.NET Core microservice template which compiles to fully native binary, it also does not use any special tricks - just regular code).
They are very much not 1:1 languages. Depending on the domain, there may very large differences in developer productivity, level of comfort and effort required to achieve a competitive implementation.
Java strengths lie in its comparatively larger and more diverse ecosystem in enterprise space alongside certain high-profile projects, predominantly by Apache foundation. On technical merits it does have less to offer.
The main technical exception - Java has superior GC implementation(s).
I'd say if your goal to expand your horizons, then it's more important to pick a problem that can't be nicely solved with Java. In that case, C# will offer more pleasant and moderately familiar experience over C, C++ or, to an extent, Rust (which is another great language to learn).