Another stat to add to the mix: Boeing has sold just over 8,000 B737s while the Airbus A320 family numbers about 6,400 (but they started about 7 years after the first 737). Both aircraft production numbers are currently about 1,000 per year. For what it's worth ...
B737 first flight April 1967
A320 first flight February 1987
B737 20 years head start.
B737 orders and deliveries about 12,800 to date
A320 orders and deliveries about 8,100 to date
Half my working life has been the window seats on B737's and as an aircraft I really like it and it is still my favourite airliner of choice, but as a passenger the A320 is far better due almost exclusively to the extra cabin width.
In the final decision it is the customer who decides the aircraft type whether it is the cabin comfort, ticket price or distance flown and the it is the airline management responsibility to make the best guess on what the customer wants to fit the correct aircraft.