well.. in a utopic gramatically correct sentence: Você prefere arroz a pão?
but even though we know is wrong, we would say: você prefere arroz do que pão?
actually, if you try to say it correctly, maybe some people won't understand what you're trying to say. so.. you're choice.
examples with both forms (right/wrong):
- I prefer rice to bread = eu prefiro arroz a pão / eu prefiro arroz do que pão
- I prefer apples to bananas = eu prefiro maçãs à bananas / eu prefiro maçãs do que bananas
(ps: use à [a+a] before feminine noun)