Use OCaml if you want easily native binaries, predictable performance and need very little in the way of well-documented, high-quality third-party libraries.
Use Standard ML if you want to learn OCaml/Haskell/F#/Scala. Why? Ultimately it comes down to usability. First off, operators (not named functions) are to programming languages as icon-only buttons are to web pages; they are completely unusable. The only reason Standard ML avoided the plague of the operators is because it's a simpler language that was used less. Second, since it's a simpler language, you get a much cleaner introduction to things like monads and modules. It makes it very simple to jump into more complex languages like Haskell or OCaml or Scala.
I work on Ponyo [0] because I want to allow people (myself) to program at a high-level in Standard ML. Furthermore, I want to promote quality and usability. Basically, every function's type is annotated and we avoid abusing operators. The current state is such that you can parse CLI args, parse/marshal JSON, serve web pages, make http requests, browse file trees, and many other ideas in the works. But the most important part about Ponyo is that it is a large, (hopefully) well-documented project that others can study to learn Standard ML (and Haskell and Scala and OCaml and F#) better.
Scala, Haskell, F#, OCaml have their place and do their jobs well. If you don't know their place or just want to get exposure to the history behind these modern languages, I recommend giving Standard ML a shot. You can get started by reading the /r/sml wiki [1] or joining #sml on Freenode.