I have an Instant Pot and a slow cooker, and was persuaded to get a Ninja 14 in 1 as neither has an air fryer.
I won’t go as far as to say I regret it, but I think I’d just have got a simple air fryer if I were doing it again. I prefer the IP for pressure cooking, and haven’t used the slow cooker function on the Ninja, as I use the single use one. I rarely cook meat, as my husband is vegetarian, but I use it for things like rice puddings, stewing fruit and veggie casseroles. My slow cooker has a bowl that can be used as a serving dish, whereas the Ninja’s bowl is just for cooking. This doesn’t matter if you are making a casserole, as it can be decanted into a dish for serving; but it is a disadvantage for anything with a topping. Also, the Ninja is big, and takes up space.
I do like the air fryer on the Ninja. Air fried chicken is delicious, and things like baked potatoes are much quicker than in the oven, but crisper than in the microwave. I haven’t had mine for long, so haven’t got used to all the functions, but I think the air fry one will be used the most.
For pressure cooking, I think the IP is great. It is very easy to use and efficient, and has other functions, including a steamer. IP also do a model with an air fryer. I wish I’d gone for that (I didn’t find out about it until too late) then I could have passed on my old one.