I don't think Buffett was comparing bitcoin to gold. He doesn't like gold investments for the same reason - it's not productive. He's talked about that a lot in the past.
http://www.businessinsider.com/warren-buffett-compares-stock...
> "The asset itself is creating nothing," said Buffett, comparing bitcoin to "productive" assets such as gold or a farm.
That actually wasn't the quote, the remark about gold and farm was made by the article's author.
Edit: here[1] is the actual quote and video