They both have mixed dependibility that differs from the type of car. Audi tt's i know have a timing belt SERIOUS problem (timing belt could go early causing you to buy a totally new engine). While i am not sure about the mazda. i own a 2005 mazda6 and so far it has been reliable (no fixes.. although i have a misterious engine ticking noise that is quote: common).
Personally i think they are pretty equal cars, and you should decide weather you want a two seater, or a 3 dr coupe.
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