multi = Multiple games using the same memory card.
There's no real advantage, except if one game unlock features by checking another game's savegames data. You need to use a very big memory card size to fit all your savegames on the same card. If the card is corrupted you lose all your savegames at once.
Individual = one memory card file created per game.
Many Advantages using individual :
- you don't have to worry about the memory card size and it will never be full. set it to 251 blocs (default size), it's big enough without taking too much size on your device.
- if a card is corrupted, you won't lose your other game's savegames.
- You can play japanese imported games without losing your other savegames (you can't save a japanese savegame on a card which contains non-japanese savegames. it works for few games, but not all)
You could use "multi" for all western games, and set it to "individual" only for japanese games, so they don't conflict with each other.
but individual for all of them is probably better, and if you need a game to unlock content using another game's data, you can use Dolphin emulator to copy savegames to another memory card file or copy+rename the memory card filename to the new gameID.