I actually have a first gen Surface Duo and it's held up, though its been relegated to the second device role since it stopped getting updates. Anything with an actual folding display has seemed weak IMO, I'd be afraid of breaking it or finding weird distortions in the screen after a bit of use.
The major advantage of the iPhone is that its ecosystem of cases means you can get some pretty high quality cases that can make it actually protected.
The Duo has its own faults though, the body around the charging port is particularly fragile due to how thin they made the device.