Except for certain aspects of the cockpit, the 320 is not newer tech. Several of the original NWA A320's have already been chopped up (88/89 deliveries, go figure).The 737NG series has a redesigned wing (25% greater area) that is more efficient than that of the 320 series. Taking off from equivalent runways, the 737-800 carries more passengers farther than the A320. As Sailingfun pointed out, there are many times where Delta 737-800's make the west coast out of JFK when JetBlue is fuel stopping. The 737-700 in Delta's configuration has substantially greater performance than the Delta A319 in runway allowable and climb limit weights plus range. The Airbus does have some significant customer advantages with a wider fuselage resulting in more internal space, and its cockpit is substantially more spacious.

The MD-88/90 are range limited when compared to either of the products above, which is why they've been moved from SLC to MSP.