I had access to performance data for the A319 with CFM's and the 737-700 with winglets a few months ago. I posted the findings on A-net, perhaps you could find it with a search.
This comparison only address fuel burn for a given payload, obviously not the total cost.
The 737-700W was about 1% more fuel efficient at high gross weights vs. the A319 and 5% more efficient at higher weights due to the fact the 737NG can climb to FL410 as it gets lighter, but the A319 max certified altitude is FL390.
Other factors, the 737's engines are slightly less efficient because the bypass ratio is less to let the fan clear the ground on the low 737. This is apparantly made up, and then some, by the fact the 737's fuselage is smaller and hence lighter.
The best attribute of the 320 family is it's wider fuselage, so airlines don't mind giving up a little fuel for the extra room.
The best attribute of the 737 is it's range. With 47,619 pounds of fuel, 8.5 hrs + IFR reserves, standard, more on other versions; it can easily fly US transcons with a decent payload in constant 180kt headwinds.
I fly really fast and take a lot of chances.