I prefer em because you leave the default size choice to the user viewing your pages and set offsets from that as desired. Still, web page developers could do something stupid like make their text so small that I have to then enlarge all of their pages. In the last couple of years I've noticed many sites making the text very small so that it looks pretty on their layouts. This is becoming very annoying. Sometimes I find the text size in these very threads too small. Hmm. Maybe age is catching up to me?
Here is a nice relatively short explanation;
CSS Font Size Explained.