Which is more productive, which yields higher quality?
I have build plenty of software both ways. And my answer is a confident "it depends". But I will tell you this: A fine-grained type hierarchy over either Markdown or HTML opens up a very big can of worms.
All this reminds me of SOAP vs. HTTP/JSON: I have seen plenty of SOAP webservices seriously stuffed up regardless of/because of strong typing whereas developers seem to do much better with HTTP/JSON because of the simplicity and directness of the approach.