64 bit is not really necessary for the Wii U since it only uses 2GB of RAM, when you want to use more than 4GB of RAM then 64 bit will be needed and the PS4 uses 8GB, so that makes sense. 32 bit versus 64 bit is not really exciting, except for the RAM usage (and some other small things that are not really important for consoles). The only issue it could present is ports from the PS4 to the Wii U.
And I want to emphasize on the x64 term, x86 can be 32 or 64 bits, x64 is just a marketing thing.