How they’re alike: you can cook things without heating up the kitchen. They are closed during the cooking process; you can’t stir them while they’re running.
Other than that, they’re not alike at all. The air fryer uses hot air that surrounds the food. The instant pot relies on pressure.
I use my air fryer to make foods that are breaded without frying them in oil. Also fast baked potatoes and individual pizzas. The air fryer will make crispy foods. It’s also good for making chicken breast meat for salads or soups.
Instant pot makes food FAST. It does not make it crispy at all. You can make dried beans in less than an hour. I use mine primarily for shredded chicken or pork, taco meat or spaghetti sauce, and dried beans. Baked potatoes and the chicken for salad or soups also possible in the instant pot.
I have a tiny kitchen, so I have a three quart IP which lives on a shelf until I need it. The air fryer lives and works on industrial shelving next to the kitchen. Keep your eye out at Goodwill. I see air fryers there a lot.