This is the GMC "local" city bus. It has two sets of doors (front and rear), is lower to the ground, and has ads on the sides. This is the type of bus that has low, plastic seats, grab bars for standing riders, and wide walkways.
This gizmo attaches into the front shell of the GMC Local bus. It contains the steps up, the drivers' seat, the dash-mounted bus number, and the metrocard reader.
This is the MCI "express" bus. It has the classic "ridged metal" lower half . This type of bus usually has high-seatbacks inside and is meant for longer trips with less stops.
Here is a close up of the front of the MCI "express" bus
Here is a picture of the backs of the two buses together.
This side view shows you the relative scale differences in the two buses.