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AU will add you without being responsible for the debt, joint accounts you would be responsible and would probably have to fill out an application and get a HP.
Being an AU, they just go by name and probably wouldn't know your past history whereas, a joint account they would.
For the card to work best for you it needs to have a history longer than yours and have an excellent payment history. You instantly gain the cards history, AAoA, etc.
Some AUs don't report at all so make sure the one you are added to does. I believe it is your name and address. They should be able to match you up with that information.