I thought about buying the Simplicity power head for my central vacuum because of its metal brush roll along with its beater bars, but I wanted to know would it really make a difference in the overall dirt pick-up.

Tacony's Riccar and Simplicity line have very good brushrolls in their cleaners.  The three rows on the metal brushroll will clean very well.  That does not mean that the two row brushrolls will not clean well.  The metal has nothing to do w/ how well it cleans.   The three rows of bristles will pick up debris faster than their two rows of bristles.
 
 
As a side note,
There are a few vacuum cleaners that do very well w/ hair removal from carpeting.  Also, Riccar and Simplicity are not the only two to utilize three rows of bristles on their brushrolls.