I have a 6.5KW generator and wonder should I use the 120V 30 amp receptacle to hook to house panel or the 240V 30 amp receptacle?
The 240-volt power supply will always be better and supply more power to your house. Please take note however you need one more rather sophisticated item it the mix to make a safe system. You need some type of a transfer switch to make your system safe and to keep your generator power from energizing the utility power grid. You can plug individual appliances into the generator just as is. But if you want to connect to the house panel, you need a transfer switch first. Most places this is the law. Without it people can get hurt.