At the end of the day cost is he over riding factor. The fuel efficiency of new aircraft like the 737 MAX and 320 neo are a must to remain competitive. Whether a low cost or a full service airline cost is still the most critical element. As discussed here
Boeing 737 Max versus Airbus 320 neo the 320 on average is about 10% heavier per seat than the competing 737 variant. That is a lot of extra weight overhead and cost. I like the space in the 320, but for economics the 737 is the winner.