Generally, the more personnel there is on a mission, the faster the mission is accomplished. So if you have to choose between alternatives, the more crew a vehicles can transport the better.
Since the number of missions is dependent on the number of buildings you have it is advised to build as many stations as possible as fast as possible. Beware, though, because with almost every station beyond the second and before the fifteenth your missions will require new kinds of vehicles, not only the basic one you have now! Here is an overview of what you will need:
  1. 4th fire station: "Heavy rescue vehicle" & "Utility unit" (US) / "Rescue support unit" (UK) / "Heavy rescue" (AU)
  2. 5th fire station: "Mobile Air" (US) / "Breathing apparatus support unit" (UK) / "Mobile air" (AU)
  3. 6th fire station: "Battalion chief unit" (US) / "Rapid response vehicle" (AU)
  4. 7th fire station: "Water tanker" (US) / "Water carrier" (UK) / "Water tanker" (AU)
  5. 11th fire station: "HazMat" (US) / "HazMat unit" (UK) / "HAZMAT truck" (AU)
  6. 13th fire station: "Mobile command vehicle" (US) / "ICCU" (UK) / "Mobile command vehicle" (AU)
You will find all of these vehicles in the vehicle market. You find the vehicle market via any fire station, when you click „Vehicle market“. It will look like this: