NationBuilder is a fully hosted CMS. We manage servers and scaling so you can worry about activating your supporters. We don't have plug-ins and instead have dozens of page types designed for action. We integrate email, texting and online donations along with our social CRM, things which have to be bolted on to WordPress, which can be quite painful.
WordPress is a great platform for blogging, and it also has a much more mature ecosystem of designers than NationBuilder - we have a lot of catching up to do in the custom theme department (if you want to build and market custom themes for NationBuilder, we want to hear from you!). If you want to run your own server and play around with plug-ins, WordPress might be right. But to get the native functions of NationBuilder for your nonprofit, for example, you'll have to integrate with other tools like DonorTools, MailChimp, Eventbrite and Salsa. Check out NationBuilder's fully integrated toolkit for nonprofits, campaigns and causes with a free trial.