There's no simple answer, but my analysis leads me to say that airstriking is probably better in many situations. Consider:
* Look at the numbers. If the cultural defense is +100%, saturation airstriking (that is, taking *all* the defenders down to 50%) will give you the exact same benefit as completely removing the cultural defense. Since that's a high number for defense, and will take a lot of bombing to reduce too. So usually saturation airstrikes are better. But if there are a lot of defenders, that will also take a lot of hits. Also, either one reaches its limit, and if you have enough strikes you want to do both of course.
* The new limits on air unit basing will keep you from getting too many air hits on one target. You will probably have to use some artillery too to get all the defenders down. Given that bombers don't die from airstriking, and artillery do die and take damage from attacking but not from reducing defense, this is another reason to use the air units to airstrike.
* But, taking down defenses gets no exp for the artillery. OTOH if you are *only* going to use them on city defenses, you only need two promotions on each anyway.
* Fighters don't get collateral damage. If you don't have Bombers yet, or enough Bombers, you should use fighters to bomb because they are way inefficient at striking.