Customer base: The group of current clients and consumers that a business serves. Can be a huge amount. Most often this base is business confidential.
http://en.wikipedia.org/wiki/Customer_base

Client portfolio: A selected examples of customers that is shown in order to brag. Sorry, let me be more serious: ...in order to show the world (potentially new customers) that the person/company is experienced. Often the most well known companies are shown in order to make potential customers think like "if they buy your service, I can buy your service". The portfolio can be shown on the homepage or elsewhere.


--------------------------------------------------
Note added at 51 mins (2008-11-26 13:46:44 GMT)
--------------------------------------------------

The customer base normally has information like customer's name, address, phone numer, email address etc.

The portfolio can be a selection of work, or it can include the work you have done for your customers/clients.
"selected examples, such as drawings or photographs, that show an artist's recent work "
http://www.thefreedictionary.com/portfolio