This depends entirely on your preference is or what pre-existing fuel type you may have already available at your home.
In general propane is a slightly more commonly available and customers can contact their propane provider directly to upgrade the size of the fuel tank and make arrangements to have a line added for the generator. Initial installation costs for the propane line tend to be a bit cheaper but propane is more expensive than natural gas over the long term if you will be using the generator for extended periods of time. Additionally the size of the fuel tank will determine how long your generator can run during an outage.
Natural gas installation costs a bit more but the fuel cost of natural gas is generally lower. As long as the supply of natural gas is uninterrupted your generator will continue to work.
Your generator will perform well on either fuel so many people select the fuel that they already use in their home, especially if the generator is intended for emergency back-up power and will not be used on a regular basis.
Both fuel types require a two-tiered inspection from county permitting offices. One when the fuel line is prepped and ready to connect and a second final inspection once both the generator and fuel source are connected