When comparing Scala vs Crystal, the Slant community recommends Scala for most people. In the question“What are the best server side programming languages?” Scala is ranked 8th while Crystal is ranked 24th. The most important reason people chose Scala is:
The immutable values make it perfect for working with concurrency
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Pros

Pro Immutable values
The immutable values make it perfect for working with concurrency
Pro Multiparadigm
Scala supports both Functional and Object Oriented styles of programming. Beginners can learn both paradigms without having to learn a new language, and experts can switch between the two according to what best suits their needs at the time.
Pro Type inference
Scala offers type inference, which, while giving the same safety as Java's type system, allows programmers to focus on the code itself, rather than on updating type annotations.
Pro Compiles to JVM bytecode
Aside from Java itself, Scala is by far the most popular of the many JVM languages. If you're developing for Android, or a similar JVM-only platform, or otherwise need out-of-the-box cross-platform compatibility, but the performance of a compiled language, Scala is the way to go.
Pro Very good online courses
On coursera you can find great introduction to Scala by Martin Odersky.
Pro Type inference leads to a simpler syntax
Pro Expressive functional programming abstraction for reusable and safe code
Pro Elegant syntax as Ruby
Pro Fast performance
Pro Has co-routine
Pro Compiles to native binary
Cons

Con Can be intimidating for beginners
Scala is an industrial language. It brings functional programming to the JVM, but not with a "start small and grow the language" perspective, but rather a very powerful language for professional programmers.
Con Way too complex for beginners
Even for seasoned programmers it's a difficult language.
Con Static type system inherits cruft from Java
The type system is too complicated yet still less powerful than Haskell's.
Con Aruguably no better than a meme language
The elitest attitudes in the community are unwarrented. Not recommendable for production use. Poor error messages, odd compiler errors, poorly documented behavior, etc..
Con Small community
In regards to its age and already past 1.0, the community is still too small.
Con Not cross-platform
No official Windows support.
Con A language only Ruby fans can love
Ruby-ish language.
Con Slow compilation
Con No parallelism (yet)
Not actual anymore.
