I'm much more in favour of good insulation rather than AC. Mostly because it consumes a huge amount of energy and it can contribute to UHI.

I think it is not as either-or as many commenters seem to perceive it. AC is needed specifically for extended very hot periods to bring temperatures down when it stays hot during the night. These are still rare in Western Europe.

Conversely, better insulation will drastically lower the power demand and energy costs of AC, and will also reduce the time it is needed at all. Insulation is also a mandatory requirement for heat pumps which we absolutely need in Central, Western and Eastern Europe if we don't want to give up an Climate change and accept that our civilization will die.

I think for the short-term it might be better for European cities to provide safe AC-cooled spaces im walkable distances - this is faster to deploy and cheaper than retrofit all housing. The most endangered people are old citizens and many of them don't have that much spare money.

Insulation will also help Europe to survive better if we have an AMOC collapse which could bring all of more cold, dry, and hot extremes.

Plus if AC and heat pumps are combined and integrated, soil heat exchangers can be used to reduce heating of the surrounding neighborhood which is another problem of AC.