Air fryers have become more than just a kitchen trend — they’ve transformed how many of us cook on a daily basis. With the ability to make crispy, flavorful food in less time and with less oil, it’s no surprise that brands keep pushing new designs to meet modern needs. Recently, I had the chance to try two of Ninja’s latest DoubleStack models: the Ninja SL400 and the Ninja SL451.
Both belong to the DoubleStack XL series, meaning they come with stacked baskets that let you cook multiple foods at once without hogging up all your counter space. On paper, they seem very similar — 10-quart capacity, Smart Finish & Match Cook technology, and sleek, space-saving designs. But after actually cooking meals in both, I realized they’re designed for slightly different types of home cooks.
Let me break down my full experience with these two air fryers and share which one I’d recommend depending on your kitchen style.
Key Difference Between Ninja SL400 vs SL451 DoubleStack Air Fryer
The biggest difference comes down to cooking programs and technology:
- The Ninja SL400 is a 4-in-1 air fryer with essential functions: Air Fry, Roast, Reheat, and Dehydrate.
- The Ninja SL451 expands to a 6-in-1 model, adding Air Broil and a more advanced Smart Cook System with a built-in Foodi Smart Thermometer for precise doneness.
In short, the SL400 is simpler and more budget-friendly, while the SL451 gives you more precision and versatility if you cook a wider range of dishes.
- SPACE SAVING DESIGN: Ninja’s stacked design lets you cook double the food in nearly half the space and still fits...
- SPACE SAVING DESIGN: Ninja’s stacked design lets you cook double the food in nearly half the space and still fits...
Comparison Table: Ninja SL400 vs SL451 DoubleStack Air Fryer
| Feature | Ninja SL400 | Ninja SL451 |
| Capacity | 10 Quarts (2 x 5QT baskets) | 10 Quarts (2 x 5QT baskets) |
| Functions | 4-in-1 (Air Fry, Roast, Reheat, Dehydrate) | 6-in-1 (Air Fry, Air Broil, Roast, Bake, Reheat, Dehydrate) |
| Smart Cook System | No | Yes (with Foodi Smart Thermometer) |
| Smart Finish & Match Cook | Yes | Yes |
| Design | Compact, Grey | Compact, Deep Gray |
| Meal Racks | Yes (2 stacked racks per basket) | Yes (2 stacked racks per basket) |
| Cleaning | Nonstick baskets, easy to clean | Nonstick baskets, dishwasher-safe crisper plates |
| Weight | 18 lbs | Slightly heavier (more components) |
| Check Latest Price & Deals | Buy on Amazon | Buy on Amazon |
Hands-On Review
Using both air fryers side by side over a couple of weeks gave me a clear sense of where each shines. I cooked chicken, fries, vegetables, and even tried some baked goods to push them to their limits.
Design & Build Quality
The Ninja SL400 is noticeably lighter and has a straightforward, no-frills design. It’s slim, compact, and really does fit easily under my cabinets, which isn’t always the case with bulky dual-basket air fryers.
The Ninja SL451, while still compact, has a more premium look with its deep gray finish. It also comes with extra accessories like the Foodi Smart Thermometer, which fits neatly into the unit. While both models feel solidly built, the SL451 definitely gives off a more “high-end” vibe.
- SPACE SAVING DESIGN: Ninja’s stacked design lets you cook double the food in nearly half the space and still fits...
- SPACE SAVING DESIGN: Ninja’s stacked design lets you cook double the food in nearly half the space and still fits...
Ease of Use
If you like keeping things simple, the SL400 is a dream. The control panel is intuitive, and switching between functions like Air Fry or Roast is seamless. I appreciated how quickly I could set it up without overthinking.
The SL451, however, does require a bit of a learning curve because of the Smart Cook System. But once I got the hang of using the thermometer to measure doneness, it actually made things easier. For example, when cooking salmon and chicken breast, I didn’t have to guess or cut into the meat — the air fryer handled doneness perfectly.
- SPACE SAVING DESIGN: Ninja’s stacked design lets you cook double the food in nearly half the space and still fits...
Cooking Functions
Here’s where the two models diverge the most in my experience:
- Air Frying (Both): Both models produced crisp, golden fries with way less oil than traditional frying. Even with stacked racks, the airflow was strong enough to crisp food evenly.
- Roasting & Reheating: Both handled roasted vegetables beautifully, and reheating pizza slices in them gave me crisp crusts that felt like fresh delivery.
- Dehydrating: Both models worked well for dehydrated fruit chips, though this takes patience no matter the machine.
- Air Broil & Baking (SL451 only): This is where the SL451 impressed me. I made a small batch of muffins and even tested out broiling salmon, and the results were restaurant-quality. The added versatility really does make it feel like a step up from the SL400.
- SPACE SAVING DESIGN: Ninja’s stacked design lets you cook double the food in nearly half the space and still fits...
Smart Features
The Smart Finish & Match Cook feature worked brilliantly on both. Being able to cook chicken thighs in one basket while roasting veggies in the other — and having them finish at the exact same time — felt like a game-changer for family dinners.
But the Smart Cook System on the SL451 takes convenience further. The integrated thermometer eliminated all guesswork. For busy weeknights, it gave me peace of mind knowing my proteins were cooked safely and to my preferred doneness.
Cleaning & Maintenance
Both models come with nonstick baskets and removable racks that are easy to clean. The SL451 goes a step further by including dishwasher-safe crisper plates, which made cleanup even faster. If you hate scrubbing, this is a big plus.
Ninja SL400 DoubleStack XL Two Basket Air Fryer
- SPACE SAVING DESIGN: Ninja’s stacked design lets you cook double the food in nearly half the space and still fits...
Pros:
- Slim, compact design saves counter space.
- Affordable compared to the SL451.
- Easy-to-use 4-in-1 functionality.
- Smart Finish & Match Cook for synced meals.
- Excellent for families up to 8 people.
Cons:
- Fewer functions (no Air Broil or Bake).
- No integrated thermometer.
- Slightly less versatility for complex meals.
Ninja SL451 DoubleStack XL Two Basket Smart Air Fryer
- SPACE SAVING DESIGN: Ninja’s stacked design lets you cook double the food in nearly half the space and still fits...
Pros:
- Includes 6 functions with Air Broil and Bake.
- Smart Cook System with thermometer ensures perfect doneness.
- Dishwasher-safe crisper plates for easy cleanup.
- More premium design and finish.
- Ideal for people who cook a variety of meals beyond air frying.
Cons:
- Pricier than the SL400.
- Slightly more complex to learn initially.
- Heavier build with more components to store.
My Verdict: Which One Should You Buy?
After weeks of cooking with both, my takeaway is this:
- The Ninja SL400 is perfect if you want a simple, budget-friendly air fryer that still delivers on performance. It’s great for everyday cooking — fries, roasted veggies, reheated leftovers — and takes up less space.
- The Ninja SL451, on the other hand, is for those who like precision and versatility. If you’re the type who roasts salmon one day, bakes muffins the next, and wants perfectly cooked steak without a thermometer probe in hand, the SL451 is worth the upgrade.
Personally, I found myself reaching for the SL451 more often, mainly because the Smart Cook System removed any second-guessing. But if I was recommending one for a busy household that mostly sticks to air frying basics, the SL400 would absolutely get the job done.
So, in short:
- SL400 = Best for beginners and families on a budget.
- SL451 = Best for foodies who want more control and variety.
