Conquest of a city

/\ what Jean-Luc Picard said above.

Armored Car (AC) is definitely the most cost effective for HP to absorb airstrikes (with anti-air defenses put down, which helps for any unit, ofc). MI is slightly cheaper but has quite less HP (but a bit more offensive power and 1 tile further in movement across difficult terrain than AC). I don't mind using MI to temporarily hold a city then move from there. For holding down a city in the frontline of a battlefield, AC is more efficient to me.

But in terms of nuclear strikes, I noticed that the AI tends to not nuke cities that are within their air range (the airports, not the missle silos). They will pour gold into bombings and then land a paratrooper if the city is empty. But if a city of yours is outside their air range, they will nuke it if they can but it doesn't help them to capture it. A different enemy might be able to paratroop into there from their airports though.

Just look around your city, if you already took the connecting airport network in the region, you don't really need to maintain a garrison in your cities there. I often leave my cities empty, but that's more of mid-game to end-game rather than at the start.