The A319's, in the main, operate flights with a more relaxed cabin service. i.e City destinations and shorter routes. Therefore, people aren't buying as much and hence the service is generally quicker anyway.
Not entirely true, take the operation at Liverpool, 8 A319s, 1 A320, yet a lot of the services are extremely busy, especially on routes to AGP/ALC/PMI/IBZ etc....
The arguement also that the B737 operates with 3 crew easily, so the A319 can is also not as straight forward as it looks. For example, trolleys are double carts at the front which the SCCM would need to manouvre, there fore requiring the assistance of the 2 or 3. Also on the B737, the oven, which was much larger than that of the A319, is located at the rear of the aircraft. Where as on the Airbus, its up the front, causing more problems....
Anyway, lets just see how the trial goes...