Bitcoin would not likely be the dominate alternative, however. Bitcoin exists in a niche because of coercive legal tender laws. Digital gold or silver certs would almost certainly dominate. There would be no practical reason for Bitcoin to exist.
Bitcoin's inflation rate will be lower than that of any natural commodity. Gold supply increases exponentially, while bitcoin supply will stop increasing one day. I'd consider bitcoins better than natural commodities for this main reason.
If there's something that
might eventually beat bitcoins in a free-society, it's centralized digital currencies which would work similar to bitcoin, but without the expensive mining process, as there would be no need for a decentralized record keeping. But I'm not sure... centralized solutions are dangerous, there's the trust issue etc.