Depends what costs you factor in down the line. E.g. I remember reading a study about Paris, that looked at the impact widespread AC use would have on the ambient temperature of the city. The optimistic guess was a rise of about 5°.

Where I live (Central Europe) air conditions are the total exception. Adding AC to flats is rarely ever done, thermal insulation much, much more often, even in old buildings. Maybe this is because the winters also get cold so thermal insulation has the extra pro of reducing the heating bill.