Font size in CSS - % or em?

There's a really good article on web typography on A List Apart.

Their conclusion:

Sizing text and line-height in ems, with a percentage specified on the body (and an optional caveat for Safari 2), was shown to provide accurate, resizable text across all browsers in common use today. This is a technique you can put in your kit bag and use as a best practice for sizing text in CSS that satisfies both designers and readers.

From http://archivist.incutio.com/viewlist/css-discuss/1408

%: Some browsers doesn't handle percent for font-size but interprets 150% as 150px. (Some NN4 versions, for instance.) IE also has problems with percent on nested elements. It seems IE uses percent relative to viewport instead of relative to parent element. Yet another problem (though correct according to the W3C specs), in Moz/Ns6, you can't use percent relative to elements with no specified height/width.

em: Sometimes browsers use the wrong reference size, but of the relative units it's the one with least problems. You might find it interpreted as px sometimes though.

pt: Differs greatly between resolutions, and should not be used for display. It's quite safe for print use though.

px: The only reliable absolute unit on screen. It might be wrongly interpreted in print though, as one point usually consist of several pixels, and thus everything becomes ridiculously small.

Both adjust the font-size relative to what it was. 1.5em is the same as 150%. The only advantage seems to be readability, choose whichever you are most comfortable with.