Should banks hold cocos or other kinds of buffers?

At a recent conference on international banking1, a question was posed to an expert panel – which included regulators – as to whether regulators prefer buffers or contingent convertible bonds (cocos) to satisfy capital requirements. The short answer, without much further discussion, was buffers, because they are more flexible. Of course, the comparison between cocos and buffers is specious, because the former are a type of security and the latter are a balance sheet distinction that can be sati