Unfortunately the Wii U version on Zombie mode only supports local 1-2 players (360 I know supports up to four). This is regardless of what controller setup you use. I tried with the Wii U GamePad, Wii Controller/Nunchuk, Wii Classic Controller Pro and no matter what 1-2 players on Zombie Mode.
The Wii U version has it's postives and negatives. The biggest positives is multiple control modes and off screen GamePad playback on ALL modes. Which is nice. The negative...some framerate issues, downgraded or missing features, no dlc yet. As far as graphics...eh...360/Wii U are all I tested and I think they look identical. If the Wii U looks better, its nothing significant. Wii U loads a little slower but nothing major. Also when it comes to playing online, alot more players available on the 360/PS3 versions, which isn't a surprise.