SD stands for "Secure Digital", SDHC stands for "Secure Digital High Capacity", SHXC stands for "Secure Digital eXtended Capacity". From the names only you can tell he main difference in all three i.e. storage capacity. SDHC cards offers between 4GB to 32GB and SDXC offers more than 32GB.
i have a 128gb card,cause i used it before the usbloading for loadiine,and its easy for usb install,copy it full with games to install to hdd.
for hacking the wiiu u dont need a big card.
but see what is the game with the most gb you want to install,nba 2k13 is the biggest game (21gb)
so i should take 32gb.