Commercial designers may have a choice, but there's little doubt that "em" is appropriate for accessibility. Work for a government agency? It's em for you.

Yep, we have to use ems for various government-related sites in my day job. They're OK to get working reliably as long as your pages aren't a mess of inconsistently nested tables.

If using small em values be careful, as some users have their browser's text size set to Smaller, which can make sites unreadable. There's a link from that article to a script I've devised that sometimes helps avoid text getting too small.