Should I Buy Games on Windows 10 or Xbox One?
It doesn't matter. All Xbox Play Anywhere video games support full cross-buy functionality, which means that if someone purchases an Xbox Play Anywhere game on their Xbox One console, they will automatically gain the Windows 10 version for free. However, they must use the same Microsoft/Xbox account on both their console and PC.
The reverse is true for those who buy a title on their Windows 10 device in the Windows Store
app. There are no extra steps necessary beyond buying the game, and it doesn't matter which device you make the purchase on.