I think is both, asset and currency.
It can be used and works as currency currently on Japan (official currency) and asset because it has value agains't everything.
Actually the reality is that it is a currency and it was invested to be used as a currency for all of the online dealings but as the price of bitcoin is increasing very faster and the investors and traders are earning a high income when they invest in it so they take it also as asset for their investment. So for the time when it will give profit, it will also be used as asset along with the use of it as a currency.