From what I understood we can improve only one thing per city? Because I had improved the hotels of my city of Perpignan and I then wanted to improve the factories but it canceled me the improvement of the hotels. So I lost a little more than 1400 OLY, It's nothing serious but it's annoying.
Dear FloDu66,
I'm sorry to hear you've lost OLY this way. Perhaps we should think about adding a "warning" in case an upgrade already exists.
Factories can perform better for very small cities, but in most cases Hotels and Offices perform better, although they cost more.
The exact formula can be found on our whitepaper.