We're talking about the Osgiliath scene in the Extended Edition, right? The flash-back in TTT? Because I always thought that Boromir had the finer armor.
Boromir's armor may look like a common soldier's, but it's also much more protective than Faramir's wimpier leather jerkin. I always thought of Faramir's armor as what a scout might wear. It's less protective, and lighter. Boromir's armor seems more like a mighty warrior's: big, shiny, protective, and generally mightier looking. And Denethor would give his favorite son better armor, right?
A nice side-by-side comparison