Earlier quoted context omitted.
Iterate faster != Rapid prototyping. The type system definitely helps with long term maintainability (and therefore fast iteration) but when your goal is to whip out a Proof of Concept for a greenfield project Ruby wins for me hands down. (Provided it isn't overly complex, defined as "can be completed in an hour or two").
Can you explain how types slow you down for whipping up a proof of concept?
I say this not just from personal preference, but I've proctored many timed coding challenges that were language agnostic. The challenge involved consuming data from a handful of API endpoints and compiling a response. Most statically typed solutions took twice the time of a dynamically typed one. We catered the time limit to the static case and it was a simple pass fail so no bonus points for doing it faster, but there was a strong advantage to the dynamic languages.
Note: This is very much alleviated with typed interchange formats like protobuf.