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While Sony has done a good job of creating an ultra-thin 7.9mm depth on the Xperia Z, it pairs it with somewhat strange design choices, like making the power button on the side of the phone a large silver button that protrudes from the side of the device. I'm not really a fan of that (or the small flaps you need to open to access the SIM, microSD and charger sockets which are required to ensure it stays water and dust resistant) but it does at least provide minor differentiation from other handsets and also ensures the button remains accessible when it's being used in a case.
Still, I prefer the narrower and slightly shorter chassis on the HTC One, which makes for a slightly more comfortable holding position. It's not that the Xperia Z is particularly wide, it just has very square edges which don't make for the most comfortable holding position.
The One's official depth is 9.3mm and it weighs 143g, making it 3 grams lighter than the Xperia Z. The One isn't perfect though, I did repeatedly keep tapping the HTC logo at the bottom of the screen expecting me to take me to the homescreen.