Either will do the job. I would set up the Activity (committee). Then set up the sub-committees and put in the Parent code of the committee. Now you can enrol the members of the main committee, and the sub-committee. When reporting, you can choose to select by Parent code to get all the committees associated with the main committee.

Another quicker method is to use the "other" checkbox and rename the label to "Committee". Then you can report out the commitee via the Committee option in the Member report.