Performance: functions vs includes?

Is there a performance benefit or other reason to prefer including files or using functions that write the same contents? For instance, on my pages I have an number of includes for header, main nav, footer, random quote. If I need to pass variables to one of them I just set the variable right before the include. I could of course do the same thing with an include file of functions that output the data... Which one is better? Why? c -- Chris