Ambulance Station or Ambulance Extension

An ambulance station is a standalone building for ambulances and fly-cars. An ambulance extension allows you to put ambulances or fly-cars in fire stations.

If you only have two or less vehicles at a fire stations then an ambulance expansion would get you an ambulance cheaper, any more than 2 at a stations then ambulance stations are the way to go

It depends on how your Department does it
I build fire stations with ambulance expansion and move the type 1or 2 to another station because the ambulance services where I am run rescue units as well so I buy both a ambulance (however many at station)and rescue plus maybe a utility and flycar(depending if the station has them. So it's up to you
Fire station with the expansion gives more flexibility