Bitcoin is can be commodity and can be a currency too, it's depends on you used it, you look at the value of btc to fiat exchange, and you see bitcoin as commodity which only can exchange to fiat only, you need see is widely, now a lot merchant or company accept bitcoin as payment processor, it's prove if bitcoin can be a currency.
Do you know what currency makes currency or funds for that case?
The fact that a merchant in a particular country has the legal obligation to accept it. That simple fact above all the rest makes it currency/funds. This means that a merchant in the US can not refuse to accept USD for a product or service. As to bitcoin no such obligation exists hence NOT A CURRENCY. May be a token of some sort but not a currency.
quote from U.S. DEPARTMENT OF TREASURY website about legal tender question
"Legal Tender Status
I thought that United States currency was legal tender for all debts. Some businesses or governmental agencies say that they will only accept checks, money orders or credit cards as payment, and others will only accept currency notes in denominations of $20 or smaller. Isn't this illegal?
The pertinent portion of law that applies to your question is the Coinage Act of 1965, specifically Section 31 U.S.C. 5103, entitled "Legal tender," which states: "United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) are legal tender for all debts, public charges, taxes, and dues."
This statute means that all United States money as identified above are a valid and legal offer of payment for debts when tendered to a creditor. There is, however, no Federal statute mandating that a private business, a person or an organization must accept currency or coins as for payment for goods and/or services. Private businesses are free to develop their own policies on whether or not to accept cash unless there is a State law which says otherwise. For example, a bus line may prohibit payment of fares in pennies or dollar bills. In addition, movie theaters, convenience stores and gas stations may refuse to accept large denomination currency (usually notes above $20) as a matter of policy."