reason standart library or belt which one to use and why ?
This is a good article each explaining the benefits of Belt: https://dev.to/johnridesabike/avoiding-exceptions-in-reason-bucklescript-30ii
And this article explains Belt‘s approach and why it can improve your experience: https://www.javierchavarri.com/data-first-and-data-last-a-comparison/
There are some modules only available (for now) in Js though, for instance Js.String.
The exception for using Js is when you really need the zero-cost bindings for performance.