I think you need to consider what food you eat before deciding whether you need an airfryer.
Airfryers are ideal if you are what I call a dry eater, by that I mean you often cook chicken pieces, hamburgers, sausages, baked potatoes, lumps of food. If, like me, you prefer wet food; casseroles, stews etc, then you are better off with a slow cooker.
I have got an airfryer, quite small, £20 in the sales and I really do not use it very much, for pommes parmentier, chips. I tried cooking a small chicken and roast potatoes in it, but found it fiddly and needing constant attention. In an oven I just put chicken and spuds in it and go off and do something else for an hour. It took nearly as long in the air fryer and I was constantly, turning it over to make sure it browned properly.
I am not going to say whether one is better or worse than the other. They are two entirely different appliances and, personally, I use my slow cooker far more frequently than my air fryer.