Best approach for design a save system, of an City Builder.

Hello everybody! I'm finishing a CityBuilder game for iOS, and I am in doubt of what would be the best way to implement the Save Game system. Not technically speaking, but what is the best approach:

Save data every X seconds...

or

Save the data to each user action...

or

Any other suggestions...

The game is in Real Time, then there is no "turns" to enjoy and perform the Save ...