When comparing Elixir vs Reason ML, the Slant community recommends Elixir for most people. In the question“What are the best languages for learning functional programming?” Elixir is ranked 5th while Reason ML is ranked 19th. The most important reason people chose Elixir is:
Leverages the existing Erlang BEAM VM
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Great for concurrency
Leverages the existing Erlang BEAM VM

Pro Great getting started tutorials
The tutorials are very clear and concise (even for a person not used to functional programming). Plus they are also very mobile friendly.
Pro Powerful metaprogramming
Write code that writes code with Elixir macros. Macros make metaprogramming possible and define the language itself.
Pro Full access to Erlang functions
You can call Erlang functions directly without any overhead: https://elixir-lang.org/getting-started/erlang-libraries.html
Pro Scalability
Elixir programming is ideal for applications that have many users or are actively growing their audience. Elixir can easily cope with much traffic without extra costs for additional servers.
More details can be found here.
Pro Great as a first functional programming language!

Pro Great documentation
Elixir's documentation is very good. It covers everything and always helps solving any problem you may have. It's also always available from the terminal.

Pro Syntax is similar to Ruby, making it familiar for people used to OOP
All of the benefits of Erlang; without as steep a learning curve of prolog based syntax. Elixir is heavily inspired by Ruby's syntax which many people love.

Pro Easy to download libraries
Comes with built in build tool called "mix". This will automatically download libraries and put them in the scope of the application when you add them to the "deps" function and run mix deps.get
Pro Uses the excellent Bucklescript Ocaml to Javascript transpiler
Pro Superior type inference
Ocaml type inference is so smart that you never have to repeat yourself and keep code very clean, type errors also are very pleasant.
Pro Aims to make the transition from Javascript easier
Despite being a completely different language Javascript programmer will find that the syntax of ReasonML has many familiarities with Javascript.
Pro Uses established compiler technology from Ocaml with a tweaked syntax that leans more towards Javascript
Pro Removes JavaScript "bad parts" but sticks to it's design philosophy
unlike other js-targetting languages that are thought as a way to have a language that pleases community X run in a browser, reason is really designed with JavaScript community in mind. it removes the bad parts but keeps its syntax and its best design principles (from Scheme): simplicity, minimalism, and functions as building block.
Pro Immutability with escape hatches
reason includes true immutability, but it has escape hatches to let you use mutations in exceptional cases.
Pro Compiles to JavaScript or assembly (ocaml)
The same reasonml code can compile to js (eg. run on browsers or node.js, use any lib in npm), or compile to assembly thru ocaml (unless of course you load js externals), running on any device, with C-comparable (or better) performance.
Pro JSX syntax natively supported
Reason was created by the creator of react, for developers already using JSX to template web or native UIs this results very familiar.
Cons
Con Deployment is still not as easy as it should be
Con Some design choices may seem strange
Some design choices could have been a little more appealing, for example: using "do...end" comes natural in Ruby for blocks but Elixir uses them for everything and it looks pretty weird:
Enum.map [1, 2, 3], fn(x) -> x * 2 end
or
receive do
{:hello, msg} -> msg
{:world, msg} -> "won't match"
end
Con A standard async syntax is pending
Async syntax is not standard across native/js projects and in both cases a bit awkward for non-ocaml devs. Currently this is reasonml most voted issue in their GitHub repo so hopefully, there's news soon.
Con Ecosystem is a mess
A wonderful language, but a user is required to use multiple different package managers for many things (esy, bsb, npm). Some standard templates from bsb (e.g. react-starter) are not installable out of the box.
