Hardly wishing to take sides I would hazard the guess that AdM was referring to the comparative age of the aircraft the fleet. Some of their aircraft, although they are 320/321s, have been around a while and been sourced from different places which means their configuration may vary slightly - unless they have been refitted.
Of course perceived "age" has a lot to do with maintenance and how much use the aircraft gets. Some of the orange 319s look like they have been flying non stop for 20 years, whereas a 20 year old 320 I flew a few years ago looked brand new.
As to EZY versus MON - as an example - again it all has to do with perceived stability, satisfaction and morale. It is very subjective and personal; a permanent job in what could be considered as a less "robust" airline may be better for some than no contract in a more "robust" airline.