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.

This is about BM's suggestion expressed in the mumble hangout on Oct 23rd, regarding the idea of temporary substitution for committee members.
Maybe somebody is able to clarify this issue for me.

In the pre-hangout part BM said:

QuoteI think at this point I wouldn't object to some of the Witnesses also being Committee Members. I think that in the long run we'd want to separate out those roles, but we only have so many people in the Community with technical knowledge and right now the Witnesses are the most capable.

But later on (at 1:16:55) he said:

QuoteI think it would make sense to have the committee members and the proxy voters be one and the same.

So what is the actual intention: to have the witnesses or proxies temporarily acting as committee members?