It depends on what you mean by "better." :) The problem is they're not exactly the same thing: Groff (and LaTeX) are much closer in spirit to HTML than they are to Markdown, in that they're the final format you're going to use to render the final product. Markdown, though, was always intended as an intermediate format that you'd run through a processor to produce HTML. It's not as capable as HTML (or LaTeX or...) because it wasn't ever meant to be; it's supposed to cover the basics. MOM has macros for pairwise kerning, horizontal and vertical movements, and precisely-measured horizontal rules; those are things that are way outside Markdown's scopes.
As for whether or not MOM is easier than Markdown for "first draft" writing, though? To pull just a single bit out of a FAQ that I wrote recently, in Markdown I wrote:
**That's pretty cool! What about, uh, not movie theaters?**
The home version of Atmos works _essentially_ the same way, just
mapping to fewer speakers---typically just five, although you can
get fancy and go up to eleven.
From what I see, in MOM, that would become:
\*[BD]That's pretty cool! What about, uh, not movie
theaters?\*[PREV]
The home version of Atmos works \*[IT]essentially\*[PREV] the same
way, just mapping to fewer speakers\[EM]typically just five,
although you can get fancy and go up to eleven.
So when it comes to ease of writing, the thing Markdown was specifically aimed at, I'm gonna say it definitely has the edge. If I was going to typeset a book in Groff, I'd be down for writing a Markdown-to-MOM translator, which seems like it'd probably be pretty easy. But I wouldn't start out drafting in MOM any more than I'd start out drafting in LaTeX. Or HTML.
