(a) A member of the Commission who is an officer or employee of the United States shall serve as a member of the Commission without additional compensation, but shall be entitled to reimbursement for travel, subsistence, and other necessary expenses incurred in the performance of his duties as a member of the Commission.

(b) A member of the Commission who is not otherwise an officer or employee of the United States shall be compensated at a rate of $150 per day when engaged in the performance of his duties as a member of the Commission, and shall also be reimbursed for travel, subsistence, and other necessary expenses incurred in the performance of his duties as a member of the Commission.