Baroque, do you believe that the alleged desire to served small airports and the runway performance of the MAX -7 may be the reason for Jetline's choice of equipment ?
I do not know specifically how the 737-7 compares to the A319. However, I do believe the A319 is quite a strong performer and has been used in some of the world's most difficult missions.
Here's a video that may be of interest. A Druk Air A319 landing at Paro Airport, Bhutan which lies at 7333 ft. ASL with a 6445 ft. runway. Of course, we don't know the payload, but it just shows how nimble this beast is. It's final approach at 6:00 is absolutely crazy how you don't have a clear view of the runway until you snake your way around the two mountains in its line of sight.
I bet this airline carries extra barf bags on board...
The press release states that this is not a firm order yet ("The order will be posted on Boeing's Orders & Deliveries website once all contingencies are cleared"), and that Air China's commitment is for both NGs and MAXes. This seems to be a consequence of the airline's choice of delivery dates.