Hi! I'm not familiar with Ex Novo, so can't compare them, but Polis is really an engine for building and exploring your city. The mechanics are crafted to create an interesting and enjoyable interaction between the physical geography of the settlement, the external and internal forces at play in the city, the culture and factions in the city and the decisions made by the people of the city. It guides you to introduce events and changes and then react to them, before diving into roleplayed scenes which might generate more changes in turn.
It contains a lot of examples but no tables, so yes it's primarily down to your imagination. If you're looking to throw together a city from random elements, this probably isn't the right game for you, but if you like the idea of playing through the development of your city organically, bouncing off the contributions of the other players at the table, you'll have a great time with Polis.
