Well if you think about it, it's a little of both. It is a currency only because it has a monetary value attached to it as well as having online and I think maybe even physical ( I could be wrong on that ) vendors that accept it as payment for goods and services. It also serves as an asset as it periodically rises in falls in value. There aren't many things that are like this and it's just one more unique thing that bitcoin has going for it. I think one day it will lean more towards the currency side as it will have a fixed value or something close to it.