The HTC U Ultra is 6-percent taller, 7-percent wider and 54-percent thicker.
Note, though, that the Moto Z gets thicker once you snap on any of its Moto Mods. (Read on.) Also note that we're only covering the standard Moto Z and Moto Z Droid (Verizon version) in this article. The Moto Z Force is its beefier fraternal twin, with better camera, bigger battery and higher price.
(There's also a cheaper, mid-ranged Moto Z Play that's a completely different phone.)
The Moto Z is 20-percent lighter (though that's also without any Mods attached).
HTC stole a page from Samsung's playbook, with its glass and aluminum design for the Ultra: From the back it practically looks like a spitting image of a 2015-16 Galaxy S flagship.
The Moto Z has the best approach to modularity we've seen, as you can snap on a Mod (sold separately) to add things like extra battery, camera with optical zoom, JBL speaker, projector or just a new look and material.
On one hand, the Mods add versatility to the phone, and it's by far the best approach we've seen to smartphone modularity. On the other hand, several of them have questionable utility (a projector on a phone?) and it certainly adds complexity and cost to the typically-simple and streamlined smartphone.
You have three color options to consider for either phone.
The Ultra has a 7-percent bigger display.
Both have ultra-sharp QHD (1440p) screen resolution.
It's IPS vs. AMOLED.
HTC also appears to have copied LG, with its second screen. It's home to shortcuts and notifications that don't intrude on the primary screen area.
Despite launching eight months after the Moto Z, the HTC U Ultra only has a half-generation-newer processor. Qualcomm's latest silicon is missing from HTC's flagship.
RAM is tied at 4 GB.
HTC gives you the more generous base internal-storage tier. (In its case, it's the only tier.)
Both, however, have expandable storage.
The Moto Z's internal battery is smaller – part of the reason it's so light and thin – but you can add a 2,220 mAh battery-pack mod (if you don't mind paying).
The only notable difference here is the HTC U Ultra's surprisingly high-res front camera.
Camera aperture (rear)
Aperture, which often weighs into low-lit photography, is tied.
Both have fingerprint sensors below their screens, though, somewhat unexpectedly, the Moto's isn't a home button. (Instead it's a wake-sleep button.)
Both run Android Nougat, but HTC's has a custom UI. The Moto Z has a few extra features thrown in, but is otherwise stock Android.
If Lenovo goes with a one-year release cycle, we may be less than six months away from a Moto Z follow-up.
HTC announced the Ultra in January, but it doesn't start shipping until March – a long wait in the rapidly-changing world of smartphones.
Starting price (full retail)
Be sure to factor the cost of Moto Mods into the Moto Z's price: They can add up quickly.
For more, you can read our full review of the Moto Z and Moto Z Force.