Wouldn't mind a Pokemon main game on the Wii U myself, but I'd like to see Z on the 3DS.
For the Wii U they could do a much more expansive game, featuring much more regions than a handheld version could have. Though there would be some issues regarding pokemon being overpowered after a couple of regions completed, I'm positive Game Freaks could overcome them.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"