I used the OG xbox as my main device for many years, the homebrew scene is much larger and the Wii-u scene is kinda sad in comparison.
but I think that the Wii U has a lot going for it, it's now my main device.
Wii-U positives:
The N64 and DS injects work much better than any sort of N64 and DS emulator on OG Xbox.
The Wii-u Pro controller has a much nicer D-pad that OG xbox controllers.
Retroarch has netplay (yes XMB front end works)
Native Gamecube and Wii backward compatibility
Console fan is much quieter
OG Xbox positives:
Much more mature and diverse emulator homebrew especially for 16bit and older consoles
Decent Playstation One Emulation
Internal harddrive