There seems to be a preference by those with a bent for css to prefer the 'em's and they are a recommended 'best practice' by w3.org (
http://en.wikipedia.org/wiki/Em_(typography) and
http://www.w3.org/WAI/GL/css2em.htm).
I have gotten comfortable with 'em's and fwiw at this point I find them convenient for describing font related sizes like font-size and line-height and font proportional padding/margins.
I found two more interesting links discussing the matter here
http://css-tricks.com/css-font-size/ and here
http://www.w3.org/Style/Examples/007/units.en.html.
The line of business connected to this forum has, how should one say (?), an emphasis on image presentation and web image sizes are described in px. This may more often make px a more intuitive measure here...