I'm a developer on CruiseControl and I work for Urbancode who makes AnthillPro, so I can speak to the strengths of those. I'm also one of the organizers of CITCON so I'm pretty familiar with almost all the offerings in the space.
CruiseControl or Hudson are both pretty good open source CI tools. I find CC really easy to customize and extend and there are lot of plugins that have been added over its 8 year lifetime. Hudson is newer and easier for most people to get started; it is dead simple to get your first project running using the GUI configuration while CC requires mucking about in XML files. Hudson also has an active plugin community and is rapidly adding new capabilities.
However depending on the complexity and requirements of your projects you might find a commercial tool like AnthillPro would be a better fit. The reasons upgrade from CC or Hudson to AHP vary from company to company but some typical answers include the ability to setup self-service deployments with access control by environment, the desire to chain multiple builds/workflows together, the built in dependency management, the artifact repository, or the ability to pull together data from a bunch of different tools & builds across the lifecycle.
Not everyone needs those capabilities but the people who do find the upgrade to be worth the money.