When comparing PureScript vs Standard ML, the Slant community recommends PureScript for most people. In the question“What are the best languages for learning functional programming?” PureScript is ranked 10th while Standard ML is ranked 15th. The most important reason people chose PureScript is:
Has Typeclasses and RankNTypes
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Higher kinded types
Has Typeclasses and RankNTypes

Pro High performance FFI code
The Eff monad, which is used for FFI code, optimizes out calls to bind, and supports tail call optimization, resulting in clean, efficient Javascript. The psc compiler also specifically recognizes the ST monad, and transforms scoped variables into mutable Javascript variables, for even more efficient code.
Pro Type safety
Compiling should be your first unit test. A tight type system (static and hopefully strong) will catch many logic errors that are often difficult to spot through debugging. In languages like PureScript, if it compiles, it often runs properly.
Pro Pure functional language
You cannot have side effects, unless a function is explicitly defined as so.
Pro Modules can be compiled to CommonJS
Modules compiled to CommonJS can be included with 'require', making it incredibly simple to call Purescript code from Javascript.
Pro Has row polymorphism and extensible effects
Pro FFI
FFI system is quite good and easy to use. You can import functions curried or not curried. Records and arrays use native JS objects and arrays.
Pro Thorough documentation
The Purescript website has fairly thorough documentation for all of the language's features, and the Purescript blog contains several examples of practical usage.
Pro Awesome web frameworks
Thermite (React)
Halogen (VDOM, similar to ELM)
And hit these up with Signals, Isolated/(Managed?) Components, powerful functions and FFI
Pro Powerful module system
The module system that Standard ML uses gives the programmer the power to define custom data types whose internal implementation is invisible to other programmers using the module.
Pro Implementing laziness is trivial
Since mutability is only confined to a special type of reference cells, implementing laziness in SML can be done in only 20 lines of code.
Pro Enforces distinction between data and computations
Since it uses strict evaluation, it enforces distinction between data and computations which in turn enables you to use induction on algebraic data types as a reasoning principle.
Pro Great exception system
Secret messages can be sent across distant parts of a program without possibility of being intercepted by unintended recipients in the middle.
Cons
Con Lots of dependencies needed to get started
Purescript is written in Haskell, but meant to be used with Node.js. As a result, to get started , users must install ghc, cabal, node.js, grunt, and bower. Purescript also has its own compiler, and different semantics form Haskell, and so even after installing, there's still some overhead to getting productive with Purescript.
Con Lack of good IDE/tooling support
Con Documentation not updated
Con Ecosystem not stable
Con Restrictive FFI
Functions exported are all curried, and must be called as such from Javascript. The FFI syntax for importing Javascript functions, while slightly simpler and more readable than UHC/Fay's, means that calls to methods on objects must be wrapped to pass the object explicitly as a parameter.
Con Slow compilation
On large project, for example Halogen
Con Not very popular outside academia
SML is mostly used in academia and doesn't have many uses in industry. While it's a good language for learning functional programming concepts, the language itself won't be very useful.
