The Wii is a fairly powerful console, and a softmodded one does lend itself well for emulation. However it is a fiddly UI to use for emulation that is based on keyboard/mouse based systems. Also I'm never been 100% happy with emulation for most systems. SNES emulation is very nice and you can tun Gamecube and N64 games natively so it is a great gaming system anyway.
The original Xbox might be a bit less powerful, but is was adopted by the emulation scene more than any other console to date because it is fairly easy to port emulators due to its architecture and they are quite advanced and easy to use.
The Xbox is also a lot more advanced for anyone whom likes to play around with a system as you can easily network it to your PC to transfer files and play around.
The Wii is also a bit limited in terms of storage compared to the Xbox. Although it is easy to just put files on an SD card, which isn't possible on the Xbox. You also have the homebrew network for downloads on the Wii which is a very nice resource.