Things to consider:
Natural gas / city gas is wonderful if you have it. BUT consider what happens when a large storm comes through.
Trees are knocked down, when the trees go down, they rupture water lines, gas lines, electric power lines etc.
I've seen this here in Ft. Lauderdale. Propane is wonderful if you have a BIG propane tank on your property, not so much if you will feed the genny with individual cylinders. Like where will you get the cylinders refilled when power is out everywhere. You gonna have a dozen propane cylinders for a weeks supply?
Gasoline is difficult too. Storing gasoline demands additives to keep it from going stale, usually good for 6 months to maybe a year. But it is easier to get gasoline in difficult times than propane. The good part of gasoline is that it can be used in the lawn mower or the cars when its time has come, or you gotta boogie on out of there.
No matter what you buy, make certain that you install a load bank to transfer power from utility to genny. DO NOT think you can get away without it.
You will need enough power to run water pumps, refrigerators, water heaters, interior lights, radio, TV, computers, small appliances etc.
Perhaps not all at the same time. 8 ~ 10,000 watts will generally do that. With a good genny, running 12 hours a day, intermittently, 6 gallons / day will suffice for fuel.
Another thing to consider, The damn things are noisy! NOISY I SAY! Beware of that in what you buy. The quieter the better.
With portables, a negative factor is that they are portable. You MUST have means to secure your genny, and I mean SECURE YOUR GENNY. If not someone else will be enjoying the benefit of your genny. Understand when the power is out, everyone wants to either take your genny or to enjoy the benefit of your genny.
I speak these truths as a long time south Florida resident with a good Yamaha genny. Been here, seen the whole enchilada and learned a bit.