So basically the sd card has a number of sectors on it in with your data is stored much in the way a CD has a number of tracks. The cluster size dictates how much info each sector hold so 64kb cluster size holds 64kb of data in each sector and 32gb holds 32gb of data. So a card formated to 64kb will load quicker as each sector holds more information and less sectors are being read by the wiiu.
Okay that's more detailed, so then why is there a option for it? If the 64k is clearly faster and more prefered, why woud options like 32 even exist in the first place?