Bitcoin works best at both but it is more recognized nowadays as an asset rather than a currency. Mainly because trading it yields profit to the investor and sometimes selling it also does. However, bitcoin also works good for buying goods and services that are provided on the internet obviously because it was meant to be a currency.
I think bitcoin can be both an asset and a currency because it has both functions as an investment, but at the same time acts as a medium for trading and its price volatility.