The MD80 is a dying breed. American (largest operator) is phasing them out with 737-800s. The 717 is the latest and greatest from Boeing right after the bought McDonnell Douglas. The 717 is a DC-9-30 airframe with a glass cockpit and high bypass (read: Efficient) Rolls Royce/BMW turbofans. It is a very economical airplane. The MD90 is not a very common airplane. It features the MD80 airframe with updated avionics and the same engine as the 717. More economical than the MD80, but less up to date avionics wise.