If you have the money, having a whole house installed, sized and all that jazz. Would be your best bet. You already have natural gas, so that is easy enough.
If you are going with a portable, they have pro and cons. For myself, the run time and the cost of propane at the time, shorter and more expensive then gas. So I went with gas. Many communities require them to be hooked up by licensed professionals now. No homeowner mickey mouse, so factor that into your cost. My town, won;t allow them to be hooked up straight into a panel even with panels that have interlock systems. So look into that and factor transfer switches into your cost. The reason is too many take shortcuts, wire them in through their dryer circuits or some such crap. Forget to turn off the main and the linemen can get zapped from your generator. Even though my generator won't come close to 50 amps, I wired it so I could always upgrade. I forget the exact ratings of the one I bought, but it hits 30 amps and that was what I used for my guide as to what is on and off. I have well water, so it can power that as well as the sterilizer and and such. Plus, at the same time, it can run my fridge, freezer chest, gas furnace, my fios and big screen TV(don't want to miss a game and want to be able to use the internet) and a couple of outlets we use for charging phones and such. I can live by candle light, without using the washer or dryer, or the dishwasher for a few days/week. I can always play with what is powered or not if something else comes up. I also have a mini generator that can run a space heater if I needed. I switched to electric outdoor tools, and I basically use the little one for the chainsaws or crap out in the yard that may need to be addressed.