The first one I guess. It all really depends on the physical dimensions though. I'd be happy with a similar form factor to the GPD XD since I don't really care about a keyboard. The main things I'd want for a windows gaming handheld is guaranteed x-input support so it's hassle free as is reasonably possible and hdmi output so I can work on a big screen when sorting out game settings, setting up emulators and other desktop things.
The only other important thing I can think of at the moment is that it can support as high a microsd as possible so since PC games tend to take up a lot of space.
Yup, Xinput is the most necessary thing. Might seem obvious but Chinese companies have a habit of missing important details. Absolute minimum internal storage should be 32gb too as Windows takes up a few gb and the games can be huge.
GPD XD 64gb, 3DS XL, DSi XL, GBA SP, GBBC Clone, Gameboy Pocket, PSP Go
XBox360, Wii, PS2 Slim, PSOne
HP Touchsmart 11 (AMD A6)
Voyo A1 Mini
Psion 5 & 5MX