That is a trickier and larger choice, as all 8.1 apps are forward compatible to WIn10.
SO it depends who your target audience is. I think things will become clearer towards the end of November once the Wn10 mobile rollout begins.
Personally (JMHO), since it is only trivial work to support both 8.1 and UWP. I would create a new 8.1 Universal project, store your game code in the shared project and then add a UWP project referencing the shared project.
The same code will run on all platforms. Only caveat, is that you will need to move the GamePage.XAML and APP.XAML (plus supporting code behind) in to the .Windows and .WindowsPhone Universal projects as the XAML is not supported by UWP.
I'm demoing this as part of Future Decoded, if I get a chance, I'll try and record a video showing it.