Delta flies the MD-88's on Shuttle Routes. The engines are in the rear which makes the ride very quite for most of the plane. The MD-88 uses a 3-2 seating configuration which helps because there are not as many middle seats. Also, DL does not sell Shuttle flights with first class, but rather a very generous all coach class. The Coach pitch is 36" rather than 31" or so at US.

US flies in a domestic 2 class configuration. They fly the Airbus A319 I believe which uses 3-3 seating in coach.