I would argue about that types can be used "instead" of JSON/XML. It's more a complimentary ability to that and helps to easier describe your data w/o much overhead. Consumer in the end will still consume JSON, XML, or something else. Types are used only for description of your data, not the consumption yet.
How about validation? Is it of the same level as XSD's or JSON Schemas?
Mhmhmm as types are based on the YAML language the overall syntax is validated and all examples are validated against the type definition as well. Is that what you were looking for?
If you may create an API from the ground up, when/why should you choose 'RAML Types' above JSON or XML?
See my response in the beginning. Its not an "either ... or" here. Types are definitely easier to describe your data w/o all the overhead that comes with JSON or XML.