I would take passport AND driving licence as a matter of course.
Driving licence is always in my wallet anyway (even at my age I get ID in bars in the US for example) and I simply have never trusted the notion that I don't have to show my passport at an airport.
Maybe it's because I have done so many many more international than domestic flights. Not that I don't believe all the other posters here. Simply that in the grand scheme of things 1) It's a tiny document which weighs nothing and 2) I have no problem with passport checks at airports as there are clearly these days bad people intent on doing bad things there and I'm not one of them.