Almost universally, you cannot apply to other schools for a year. In essence that would be saying "I'm really committed to your school. I just need to get my ducks in a row before I come...unless I find something else better." When you defer, you have to imagine that you're already enrolled at the school, just not taking classes for a year. If you apply to other schools, the offer of admission will be taken away, and you can pretty much bet on not getting into that school again. Your best bet it to defer, and ask about withdrawing after deferring for a year. Then if it's acceptable, withdraw first, and apply to schools again. That gives you a much better chance to get into the school you were orginally admitted to, although I won't say that no adcomms wouldn't feel a little offended and not admit you because you wavered.
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