The U Ultra is definitely a "flagship" level device, with a 5.7-inch display and top-end specs for the time. It also has a new secondary display, not unlike the LG V20, that can display glanceable information even when the rest of the display is turned off. The camera is nearly the same as that in the HTC 10 but adds new PDAF focusing. Inside you get top-end specs: Snapdragon 821, 4GB of RAM, 64GB of storage and Quick Charge 3.0 over USB-C for the 3000 mAh battery.
The U Play is a smaller version of the same basic platform, with the same quality hardware in a more compact 5.2-inch size. The specs inside take a drop down, moving to a 1080p screen resolution, MediaTek processor, 3 or 4GB of RAM and 32 or 64GB of storage, depending on the market. The U Play is targeted at specific markets at a lower price, separately from the U Ultra being marketed globally.