Which Ninja Air Fryer OVEN should I buy? 

How to choose the Ninja Oven that's right for you! Ninja Kitchen has a whole bunch of Ninja Foodi Ovens on the market and I've tried almost every type. I'll go over the pros and cons, the ins and outs of three different types and help you pick the right Ninja Foodi Oven for you.
If you need more information I've got multiple review and demo videos on each one.
1. Ninja® 12-in-1 Smart Double Oven with FlexDoor™ Model # DCT451 Prices may vary. I paid $349.99 directly from Ninja Kitchen's site. Look for sales and promo codes on Ninja Kitchen's site
Ninja® Double Oven is a "countertop oven with the power of two ovens in the counter space of one. The FlexDoor™ creates 2 separate ovens so you can cook 2 meals, 2 ways at different temperatures and cooking times. Sync both independent ovens to cook a full meal that finishes at the same time with Smart Finish™ technology. The Smart Cook System precisely monitors temperatures, so you don't have to. Plus, it features 12 cooking functions."
2. Ninja® Foodi® 13-in-1 Dual Heat Air Fry Oven, Countertop Oven SP301
I've purchased two of these units, one for myself and one for my son. He's the one who recommended it to me, a former roommate of his had the SP101 version. The regular price for this oven at the time I purchased it was $299.99 but I purchased my son's on sale for $249.99 and I purchased mine on Amazon used in VERY GOOD condition for only $169.99
From the NInja Kitchen site: "The Ninja® Foodi® Dual Heat Air Fry Oven gives you versatility in the kitchen with 13-in-1 functionality from Air Fry and Sear Crisp to Dehydrate and Reheat. With Dual Heat Technology, unlock up to 500°F cyclonic air and a directly heated SearPlate, which allow for fast cooking and restaurant-worthy results. Cook up your favorites and then flip the oven up and away to reclaim your counter space."
3. Ninja® Foodi™ 10-in-1 Smart XL Pro Air Fry Oven Model # DT251
I purchased this for $329.99 on Amazon
Description from Ninja Kitchens : "Ninja® Foodi™ Smart XL Air Oven, the smart extra-large toaster oven and air fry oven with True Surround Convection™, you get up to 10X the convection power of a traditional convection oven, so you can make quick family meals on 2 levels, no rotating required. Plus, the Smart Cook System monitors the internal doneness of your food so you’ll never under or over cook food again."