angle brackets in math and text mode

You could incorporate an \ifmmode conditional, to use \langle and \rangle in math mode and \textlangle and \textrangle in text mode.

This approach works with pdfLaTeX, LuaLaTeX, and XeLaTeX.

\documentclass{memoir}
\usepackage{textcomp} % for \textlangle and \textrangle macros
\newcommand{\qdist}[1]{\ifmmode\langle#1\rangle\else\textlangle#1\textrangle\fi}
\begin{document}
\qdist{word}, $\qdist{symbol}$
\end{document}

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Addendum -- As @egreg has pointed out in a comment and in his separate answer, using \newcommand to create the \qdist macro, as is done above, can run into trouble. It's necessary to use \DeclareRobustCommand instead of \newcommand.

I guess you don't want that the typesetting of the argument to \qdist changes font when in math or in text, so a simpler

\newcommand{\qdist}[1]{\text{\textlangle#1\textrangle}}

might suffice. On the other hand, if this appears in an italic context such as a theorem statement, the effect would be unpleasant:

\newcommand{\qdist}[1]{\textup{\textlangle#1\textrangle}}

seems better.

If you intend that in math the argument to \qdist should be some math symbol, then in my opinion you should use two distinct commands, because the situations are different and semantics of commands is important.

If you don't want to follow the advice, use

\DeclareRobustCommand{\qdist}[1]{%
  \ifmmode
    \langle#1\rangle
  \else
    \textup{\textlangle#1\textrangle}%
  \fi
}

which differs from Mico's suggestion in two important details: it will work correctly in every situation (Mico's wouldn't) and keeps the output upright in any an italic context.