Looking at it from a person who will likely be purchasing both the GPD WIN and the PYRA. Here are my likes and concerns with both devices. These are my views, likely will not be shared by everyone, I'm trying to leave just facts and less about preference or if preference hopefully the preference is a common view.
(no particular order, 4 up's 4 downs for each)
GPD WIN
Major Pros:
Real joysticks (GPD XD's were great)
Native compatibility with directx 11, OpenGL 4.3, GLES 3.0, & OpenCL 1.2 - no compatibility layers needed
Can run multiple OS's natively; Windows, Linux, and possibly other OS's (hack-in-tosh?)
64 bit compatible
Major Cons:
Bad keyboard layout in renders (talks of correcting it)
Less expandable storeage (64GB internal + up to 200GB microsd)
"Joystick Mouse mode" separate from game-pad mode via hardware switch vs soft switch on pandora and possibly pyra.
less accurate capacitive touch screen for a desktop environment which favors accuracy
Pyra
Major Pros
Storage capacity (200GB microsd + 512GB full SD + 512GB full SD + 8GB?? internal)
Great keyboard layout + game controls
Resistive touch w/ built in stylus for accuracy in desktop environment
Future upgrade ability / replaceable hardware
Major Cons
ARM linux software catalog (source code often needed for community ports) is restrictive compared to x86 windows + linux (open + closed source software compatibility often requiring no community ports)
Only native compatibility with GLES 2.0, OpenCL 1.1, & Direct3D 9 (Direct3D is not full DirectX) - requires compatiblity layers to achieve higher standards
Less adoption in chipset choice - not in as many devices with comparable chipsets in the wild restricting collaborative work on improving drivers etc.
Thicker design (less pocket friendly) - however gets some points back in having less of a total footprint (Width+Length).
speculative / subjective / temporary points
It's possible to get GPD WIN for preorder/kickstarter @ $300 USD vs a probable 500€ ($548 current conversion) preorder/kickstarter of pyra (MSRP and actual cost may vary greatly on both)
Pyra will have more targeted titles and packaging system with the option of making the device feel more like a game console + a full computer
PCSX 2 will likely run relatively well on z8500
higher battery life estimates on Pyra for comparable tasks (can't find ED's quotes)