Both, for different reasons.
Witnesses should have the skill to create and manage (create proposal and such) a committee account trough the cli, since it is not implemented in the UI yet.
Proxies should have the best interaction with the community and so they could more easily represent the community will itself.
All of that, of course, does not mean that every proxy or every witness should run a committee member... it still is a voluntary job.
It simply means that although in the long run we would want to separate out those roles, right now we should support proxies and witnesses that are stepping up as committee members also.