They're very similar. Some people prefer Standard ML's syntax, though I actually prefer OCaml's (most likely just a result of me learning OCaml first).
The module systems have some small differences, but are very similar (I believe OCaml's was inspired by SML's).
Standard ML is also standardized (hence the name), and there are multiple implementations, whereas OCaml is basically defined by its single implementation.
But OCaml does have a larger ecosystem, Opam, more features (first-class modules, polymorphic variants, etc.).
