We're closer to to Greensboro, but almost always fly out of RDU. RDU is MUCH larger than GSO, and there is much more to do while waiting for the plane.
The cheap parking is in satelite parking lots. Lot 3 is before you get to the congestion of the airport; Lot 4 is after. We usually drop the bags off, and check in, then go park the car in Lot 4 and ride the shuttle bus in.
Security takes more time at RDU, bacause there are more passengers. I find the airport to be very easy to navigate, except on a Friday evening--then it is quite crowded.