I've got a gpd win (first one) and a gpd xd+. While the latter runs android, it is MUCH more convenient. The reason: the first gpd win is like a genie granting you a wish. Sure, it IS windows 10 in the palm of your hand. With all the features. Runs your entire steam collection...but the keyboard is just too small to be convenient, neither the touchscreen nor the mouse emulation is really great (though the latter is kind of surprisingly 'okay-ish' for how awful it looks), and it comes with all the quirks that is windows ("haai...don't mind me delaying or interrupting your gameplay to download and install some shit on your preciously scarce hard drive
"). Oh, and the 64GB hard drive is...pretty small. It doesn't bother me that much because the hardware's too limited for AAA-titles anyway (it's an indie machine, really), but be warned. Still: playing actual windows games is the only trump it has, so if you're looking for an emulation machine, I'd absolutely avoid it.
(note: since I always used it occasionally, I had the choice between full shutdowns/reboots every time or face empty batteries. I suppose the startup time is less of an issue if you put the thing to sleep all the time).
I hesistated a lot before grabbing the gpd xd+, and on hindsight I really shouldn't have been. Less buttons yet an easier input device (android's much more catered to touchscreen use for obvious reasons). Controls are a bit better, but especially the near-zero loading times is awesome. You can be from unpacking the device to being in the middle of a game in single digit seconds.
I'm personally not a fan of the initial interface (I swapped it out for a more regular UI in a few days), but this is probably a plus for you too: the device came with an array of emulators and even some games on it!
(I'd note that android has some sweet games too, though. Dead cells, for example, is a BLAST to play on it
).
(EDIT: youtube-link with some screenies of newer system emulations:
).
The gpd win 2 seems like it fixes a lot of my issues with the 1, but for the price point I couldn't justifie it for myself. Ironically enough, the gpd win max (which is basically 3) is around the same price range even though it's a far superior machine. So the only drawback I can think of on the max is that it's larger, and thus less portable. Both are pretty heavy machines, though. So unless you want to 100% emulate a wii or PS2, I'd say these things are overkill.