When comparing ATS vs LEAN, the Slant community recommends LEAN for most people. In the question“What are the best dependently typed languages?” LEAN is ranked 2nd while ATS is ranked 3rd.
Ranked in these QuestionsQuestion Ranking
Pro Built-in formal specification
ATS has a theorem-proving type system powerful enough to prove that its functions meet their specifications. This happens at compile time with no performance impact at runtime. This can be used to prove that an ATS program doesn't have bugs commonly found in C++ programs, like "this function never leaks memory" or "this program never attempts to divide by zero" or "this buffer never overflows" or to verify pointer arithmetic, etc.
Pro Free and open-source compiler
The compiler (ATS/Postiats) is GPLv3.
Pro Functional programming
The syntax is ML-like with the usual functional language features like pattern matching and tail-call optimization.
Pro High-performance systems language
ATS works as a low-level systems language. ATS programs have performance and footprint comparable to programs written in C/C++.
Pro Good module system
Similar to Modula-3. This makes ATS a viable choice even for large-scale projects.
Pro Safe concurrency
ATS can prove its concurrent programs have no deadlocks or race conditions.
Pro Emacs support
Pro Visual Studio Code support
Lean is a Microsoft Research project, so they have their own IDE support.
Pro Permissive license
Pro Interactive tutorial runs in the browser
Con No Windows version
But it does run on Cygwin.