Passing command-line arguments to LaTeX document

That is, can I pass latex some command-line arguments so that I can choose which style to use based on that argument?

Yes. Three options:

One

In your source file, write

\providecommand{\comment}[1]{\emph{#1}}% fallback definition

and then compile the LaTeX document (“myfile.tex”) as

pdflatex (whatever options you need) "\newcommand\comment[1]{\textbf{#1}}\input{myfile}"

Two

Alternatively,

pdflatex "\let\ifmyflag\iftrue\input{myfile}"

and then have in the source

\ifcsname ifmyflag\endcsname\else
  \expandafter\let\csname ifmyflag\expandafter\endcsname
                  \csname iffalse\endcsname
\fi
...
\ifmyflag
  \newcommand\comment[1]{\emph{#1}}
\else
  \newcommand\comment[1]{\textbf{#1}}
\fi

Three

Or even

pdflatex "\def\myflag{}\input{myfile}"

with

\ifdefined\myflag
  \newcommand\comment[1]{\emph{#1}}
\else
  \newcommand\comment[1]{\textbf{#1}}
\fi

which is probably the shortest, albeit slightly fragile because you never know when a package might define \myflag behind your back.