This is not just a question about the best outdoor heater, it’s a question about all kinds of heaters. Both have their pros and cons. It should boil down to what you prefer and what your outdoor conditions are. Some argue that gas heaters are more powerful.
This is true to some extent as electric outdoor heaters are not powerful enough to throw heat far. At best, you can feel warm within five to six feet. Both types require very little maintenance, electric heaters even less so. Both are fairly easy to install without a professional installation is necessary. You can do it on your own by simply following instructions.
With an electric heater, you are stuck with spaces that have outlets. You cannot have it in the middle of the ground without there being extension cords. Gas heaters that use tanks can be placed virtually anywhere. You can weigh the pros of one against the other in your unique situation. Whichever shows more benefits is the best outdoor heater for you.