Basically IELTS is from the University of Cambridge in association with Australian and TOEFL is American, so it depends which way the university leans as to which they prefer. In my experience, European institutions will accept both, but the individual place may have a preference. In both cases, the specifics of the exam means that preparation in the things practised is essential, so practicebooks would make sense, though there are many for both. Amazo or your bookshop will be able to supply them. Stick to the big publishing houses and you'll get stuff that covers what you need to know.