One other consideration with gas ones and extension cords - they need to be kept outside, and running the extension cords inside will require some type of opening (door/window/other) into your house. During the winter that brings cold air inside, during the summer warm/humid air and insects.
If you live in an area where the power lines could be taken out by falling trees/branches, or if ice storms could knock out the power lines, you'll need to think about how you'll run extension cords into your home and what openings you will need to have and the effect that those openings will have. You might need to have those openings for a few days before the power can be restored.
That's because it's not something that people think about unless they've had it happen to them or someone they know well. People are thinking about the roof, the furnace/AC, the appliances, siding/exterior walls, floors. Replacing most of those (except for appliances, but refrigerators are $2K now) cost as much as a whole-home generator. That, and many realtors... have an unimpressive amount of knowledge about home ownership costs and risks and what to look for when home shopping (plus, with a shortage of homes now, people can't be too choosy.)
It's like an insurance policy. You already spend $1,500 per year on home insurance... which most people have never had to to use. Amortize the cost over the expected life of the whole home generator and factor that into your insurance costs.