CSM is currently in an early phase. I think it's only worth if your organization uses Scrum or you want get some exposure. In any case, you should go further in the scrum certifications list if you really want to use it as a credential of your scrum knowledge. To obtain it you only need to attend a 2 days training, by a certified scrum teacher, and take an online exam that, at least in April, doesn't have a minimum punctuation to pass.
Prince, I don't know exactly, but I know PMP, and my experience is that it will help you to get a wider perspective on project management techniques and terminology.
In any case, both Prince and Scrum are quite different philosophies, Prince more formal and Scrum more 'new age'. I consider though that any PM must have some formal PM training before moving to Agile-like methodologies so that he is aware what is getting simplified or minimized, because some assessments in Agile are just implicits and the points must still be considered when undertaking a project.