27 Kw is a very big unit. Are you sure you need that capacity? A smaller one will save you some money in the cost of the unit and fuel.
Yes, I can certainly get by with much less during an emergency situation. However, I'm finding that a big factor is the installation cost as both require transfer switch, wiring, propane tank, underground conduit, permits, etc.
Also... the bigger units supposedly put out less noise (water cooled vs not) and run more efficiently. So, it almost makes sense if I'm doing this that I go the full route.
Not enough info..27 kw would run almost 3 houses at one time, including the water heaters and electric clothes dryers. Diesel fuel stores for decades with proper care, and is the most efficient. Propane also stores excellent, but the generator fuel regulator can have issues if not exersized each month. Propane useage will be high compared to gas and diesel, and is less common. You need a filling station for propane, where diesel is available everywhere. Natural gas is great, as long as the pumping stations are on line. I prefer diesel with a few 55 gallon drums. Red diesel is great up here, because of the - degree temps. I have a small 5kw gas for backup, when servicing the diesel. We use ours every year living at the very end of the grid above Moyie Springs ID.