The deepest split is in form factors. While both devices offer internal and external screens, with adaptable interfaces based on how they're folded, the Ultra assumes a standard candybar layout when open, measuring 7 inches diagonally. The goal is to provide a maximally compact device that's easier to slip into your pocket or a handbag, even if it means sacrificing some functions on its external screen. The Duo, meanwhile, is shaped like a passport. It operates like a regular iPhone when closed, and it opens up to provide a matte, iPad-like 7.6-inch display. It can't literally run iPad apps, but this nevertheless expands the functionality of compatible iPhone software, in some cases enabling split-screen multitasking. If you're expecting to get more work done with a foldable, the Duo wins by default.
Performance-wise, the devices are closer than you might think. We haven't benchmarked the Duo yet, since it's not shipping until October 23, but the Ultra comes equipped with a fast Snapdragon 8 Elite processor and 16GB of RAM. The Duo will likely rank faster thanks to Apple's newer A20 Pro chip and usual iOS optimizations. For the vast majority of apps, however, there shouldn't be a huge difference.
The devices are superficially comparable in terms of cameras, with each having two rear shooters — one wide, one ultra-wide — Apple's being rated at 48 megapixels, and Motorola's at 50. The devil is in the engineering, though. iPhone cameras are typically some of the highest-rated in the industry, so we won't be surprised if the Duo continues that trend and trounces the Ultra, which has known issues versus competitors like Google's Pixel series.
On the other hand, the Ultra's front camera also slots in at 50 megapixels, whereas the Duo's is capped at 12. The Duo does have an additional under-screen internal camera, yet that may actually be worse. Apple isn't even sharing its megapixel count and is selling it primarily as a tool for FaceTime video calls. Selfie addicts are going to want to unfold and flip the Duo to use those rear sensors.
Battery and durability
The Ultra's battery lasted 31 hours in our video-based testing, and Apple claims a comparable amount for the Duo when using the internal display. Both phones should last a full day in mixed usage, in other words. Regarding toughness, both use hardened ceramic glass to protect the exposed front display; however, the Duo has a superior IP68 rating for dust and water resistance, versus the Ultra's IP48. To translate, they can both be submerged in fresh water for a short time — it's just that the Ultra is potentially vulnerable to dust particles under a millimeter. You'll probably be fine if you don't take your phone into the Sahara.