Sony Xperia X vs Sony Xperia Z5
Interface and functionality
Always one for simplicity, the interface design team at Sony delivers very familiar UIs with both devices. Suffice to say that if you've used a Sony Xperia high-end from the past two or three years, you'll feel right at home with either. And even if you haven't tried out Sony's Xperia UI before, it's likely that you won't feel lost.
With both devices, we have highly minimalist interfaces that are geared towards people who don't necessarily subscribe to the more is better mantra. Extra features, therefore, are few and far between compared to the likes of Samsung or LG. And while there have been some changes to iconography and there are some gaps in overall functionality, overall the Xperia X and Z5 offer an essentially identical experience.
Standout features include the ability to wake up and sleep the screen by double tapping the screen, a decently stocked Theme store, and the ability to quickly jump into an app search from the Xperia X's homescreen with a swipe down, a la iOS.
Finally, it's important to note that in the States, neither of these two offers a fingerprint scanner for reasons unknown. For buyers in Europe, however, that feature is available.
Processor and memory
On paper, the main difference between the Xperia Z5 and Xperia X are the different chipsets driving them. Both are Qualcomm creations, sure, but the Z5 is outfitted with the higher-end, octa-core Snapdragon 810, while the X packs a hexa-core Snapdragon 650. In terms of peak performance, the 810 obviously offers superior results, though due to thermal throttling it typically has some trouble maintaining them.
This slight superiority extends to RAM on board. In terms of size, both are fitted with 3GB, but the Xperia X utilizes LPDDR3 RAM instead of LPPDR4 like the Z5. In result, the memory is less capable, if equal in size.
Looking past synthetic benchmarks, however, you'll be hard-pressed to notice a difference in day-to-day performance. Despite its less impressive silicone, the Xperia X moves about its interface with satisfying grace, rivaling the Z5. Load times are similar, frame rate within the interface is about the same, and overall we can't complain with the X. Where you're likely to notice a difference is with games, especially AAA titles with a lot of graphics fluff. When it comes to serious gaming, the Z5 is the clear pick.
Finally, on memory, we've got a draw. Both devices come with 32GB of storage standard, and offer expansion through a microSD card.
Internet and connectivity
Sony relies on a well-established, solid solution when it comes to browsing: Google's Chrome. And it pays off.
Not only do we enjoy the cross-platform compatibility and resulting extras (such as shared history), but the client handles even heavy pages well on both devices. Better yet, essential navigational operations such as zoom and pan are handled without annoying amounts of stuttering.
As for connectivity features, the comparison isn't quite as clear cut. So while both devices offer support for 5GHz (or dual-band) Wi-Fi, NFC, DLNA, and Miracast technologies, the Xperia X pulls ahead with its Bluetooth 4.2 (compared to Bluetooth 4.1) and better LTE bands coverage. So while the Xperia Z5 can't be used on Sprint's or Verizon's LTE networks, the Xperia X can, though coverage won't be perfect—only one band is supported with Sprint, and two with Verizon, none of which the main LTE thread.
The Xperia Z5 has a few aces up its sleeve, though, with support for FM radio and MHL for wired streaming of your phone's screen to an external monitor.