Why can't it be both a commodity and a currency? People are obviously using and spending bitcoin as the companies that are accepting it are reporting quite large sales in it. Granted I'm sure most people are saving their coins but there's plenty of people who are buying and selling using them and that will only grow.
I think it can not be both because a currency and a commodity serve different purposes in which each of them is better then the other. A currency is good at buying stuff. It's durable, exchangable and so forth (see theory of money for more information on the difference between sound money and currencies, btw).
But: With more and more (hopefully) shops and businesses accepting bitcoin, it will mature more and more into a currency and move further and further away of being used as a commodity. But this will take a few years, I would say. (it took gold and silver a few thousand years, then fiat currency came along ... ;( )