[Simon]

My take on this is that unless you're creating a stylesheet that will be called when someone wants to print your page there's no good reason to use points.

Point sizes are for print medium.

Pixels are for display on a screen.

(There are also percentages and ems for sizing, but unless you're designing a "liquid layout" there's not much reason to use those.)

My suggestion is to use pixel sizes in most cases.

HTH