Specialist
It has big, beautiful 1080p display. Slim, refined design. Runs stock Android 4.4. Fast Snapdragon 800 processor. It has incredible battery life and is resistant to water.
Android 4.4 KitKat
The big ticket feature here, of course, is the unadulterated Android 4.4 KitKat. The experience is more or less identical between the various Google Play Edition devices. Everything here is stock, from the launcher to the camera. You get your typical array of Google services, relatively barren home screens, Hangouts instead of a messaging app, and access to Google Now. It is not the same experience you will get on the Nexus 5, though, as its missing features like swiping left for Google Now, or triggering an instant search by saying "OK Google" anytime the phone is on and unlocked.
Network
The Z Ultra GPE comes completely unlocked and without any carrier obligation. It supports GSM/EDGE/GPRS and LTE, so you will be able to use it on T-Mobile or AT&T's 4G networks here in the U.S. or overseas with an international SIM. Noise cancellation blocked out a wide range of street sounds, but the mic seemed to pick up wind noises too easily, and even just a hearty laugh could produce crunchy static. In my tests, the Z Ultra lasted for 21 hours of continuous talk time, which should be plenty to last you through the day.
Processor
Though it has the same quad-core 2.2GHz Qualcomm Snapdragon 800 processor with 2GB RAM, the GPE and Xperia Z Ultras turned in slightly different results in synthetic benchmarks. On the Antutu overall system test, for instance, the Xperia noted a 34240 versus the GPE's 27843. It lagged behind on the Vellamo Web benchmark, but outpaced the Xperia version on Sunspider and Browsermark. The Xperia managed slightly higher frame rates on GFXBench and Basemark Taiji, too.
Media services
There are no superfluous manufacturer media services, gratefully, and no-preloaded apps outside of the core Google apps. In fact, bloatware is non-existent. It is a breath of fresh air compared with carrier versions. And since it is only available unlocked to begin with, there was not much bloat to deal with on that particular model anyway. You cannot customize the quick settings icons in the notification tray anymore, for instance, and there are no native power saving features either. The ability to pick and choose which features you want to add, though, is among the many draws of having a stock Android device.
Apps
Even after years of deference to pure Android, it really takes advantage of such a spacious and powerful device in a meaningful way. One big thing missing here is multitasking. Sure, Web pages and graphically intensive games look incredible on the Z Ultra, but there is no real multitasking baked in here. As much as it pains me to say this, part of me misses the split-screen multitasking of TouchWiz'd Samsung devices. You can quickly swap back and forth using the Recent Apps button, but it is not quite as efficient, and have not seen any good third-party solution. This is true of the Xperia and GPE Z Ultra, though, so if you're deciding between the two, it is not a point of distinction.
Camera
The 8-megapixel rear-facing camera gets the stock Android camera controls on the Z Ultra GPE. The camera app can start up, focus, and fire off a shot at a moment's notice. Gone are Sony's multitude of scene modes and adjustable settings, but you still get control over focus and exposure compensation, and you still have an HDR shooting mode. The sensor has not changed, and there is still no flash, so images are still overly noisy or blurry indoors and under low-lighting conditions. Colors tended to run a little cool as well. Outdoors and under good studio lighting, the camera takes sharp images, but not on the same level as the Galaxy S4 or iPhone 5s.
Winding up
And the faults of the GPE are the same in the Xperia; there is no notable manner in which either device really takes advantage of the expansive screen. If your heart's set on the giant Z Ultra, the Google Play Edition is the way to go, but if you just want a stock Android device, go for the much less expensive Nexus 5 or similarly priced Galaxy S4 Google Play Edition.
Bottom line
The Sony Z Ultra Google Play Edition puts stock Android on one of the most absurdly big, but undeniably beautiful and powerful smartphones around.
Rip off
It has average camera. Lacks software to take advantage of screen real estate.