For those who want to go digital but are worried about the HDD.
You could always move all your other digital stuff to an HDD and keep only Smash Bros on the internal 32GB.
That way when you want to play 8 players, you can just unplug the HDD and run SSB4 off the Wii U internal memory. I doubt the game will take up more than 25GB so there will be plenty of space.