I know that I can use the Cloud service on two computers, one at a time. But can one be a PC and one a MAC?
Yes.
That's one of the advantages of the Cloud license over the traditional, perpetual software license.
http://www.adobe.com/au/products/creativecloud/faq.html
Are there any unique benefits that come with my paid membership?
Yes, as a member of Creative Cloud, you get many benefits that you do not get when you purchase a traditional shrinkwrapped product:
North America
Europe, Middle East and Africa
Asia Pacific