In the simplest of political theories, each party's tool is reflective of their respectivie philosophies. Talk about competitive market: RNC users compete to get points, dollars and yes, the elusive "official" GOP ipod. With what is most like an intentional emphasis on competition, MyGOP users how much money they've raised (up to $200), voters they've and volunteers they've recruited. The DNC's Party Builder is all about building and communicating within a community.
Most important is to check out how each party gathers information about the user. The RNC has different logins for different features, such as the blog, personal homepage and volunteer recruitment center. It's a model for different levels of engagement and getting lots of names without shoving committment into a user's face -- and typically getting a larger drop-off rate in return. The DNC takes a different approach. By singing up with Party Builder, the DNC gets basic information in the login and then collects information through the user's profile, signed petitions, signed letters to the editor and their network/group memberships. So why do we care? These users are the party's next loyal supporter and volunteer. And how much information the parties have on these folk will determine the strength of their online activism in '06 and '08, which is conveniently transferable to field staffs across the country.
Party Buider is superior because it uses online political activism in a social sense, not simply as a way to draw people to a website, the way EPSN or Yahoo! would. See Kari's review of the fall-down-the-stairs failure of MyGOP.
All that said, here has been my problem with Party Builder. I created an account at Democrats.org awhile back so I could comment on the blog. I tried to use that login for Party Builder, which I assume is the point.