I don't know the advantages of using px vs[/QUOTE
]pt stands for "Point" which is a unit of measurement used in typography that is equal to 1/72 inch. It is used primarily for representing the height of characters and the amount of space between lines, also known as leading. It has no place in web design and I cannot understand why it is included in Fusion.
Use pixels as your unit of measure for web sites. Use points if you're designing for pre-press output.
An em is a unit in the field of typography, equal to the currently specified point size. For example, one em in a 16-point typeface is 16 points. Therefore, this unit is the same for all typefaces at a given point size. However, the em has been co-opted for online display and is defined as the height of the font in nominal points or inches. The actual, physical height of any given portion of the font depends on the user-defined dpi setting, current element font-size, and the particular font being used. Ems are mostly used on in the web in CSS for variable size output in responsive web design. Fusion includes the em as one of its standard measurements; however, given the fact that fusion is incapable at this time of responsive output I see no reason to use it and stick with pixels as my sole measurement when designing sites.