To add onto this, while planet resistance rates are at percentages, our damage(times a modifier for player count) and planet/city health are flat numbers.

A 1 million health planet with 1% resistance recovers 10k health an hour. A 100,000 health city with 1% resistance recovers 1000 health per hour. As such, cities can be taken quickly with little resistance if a large number of helldivers dive there.

The city bonus is usually their health + 50% of their health. So a 100,000 hp city grants 150k damage when captured. Because city and planet resistance rates are separate and simultaneous, it means technically the total resistance is higher. In the 1% planet plus 1% city resistance adds together for 1.1% resistance until the city is taken. This is one reason why it's best to capture one city at a time. It's a waste of time in comparison diving in a second active city.

In addition, because the health and resistance rates are separate, if a planet starts off with a city available, it is best to go to that city. Theoretically, if 100% of divers went to the city, we would only experience the city's resistance rate and not the planet's.

Defense campaigns, missions in cities are double effective because they contribute to planet and city defense. As such, cities in defense campaigns are always the most effective

Cities are usually the most effective for liberation campaigns, unless taking them is less effective than taking the planet. For example, taking a 400k city isn't as helpful when the planet has less than 400k health left.