I ridden on some pretty uncomfortable Airbus planes. Passenger comfort has more to do with airline choices regarding types and number of seats than model of aircraft.
Competing Airbus planes have more width, which means the seats can be physically larger. Choosing different seats isn't possible in the 737: the fuselage width is fixed. The only thing they could do to get around this is go to a 2+3 layout, and they're not going to do that because the whole reason for buying and using this plane is economy.
