Prince2is on a PM level and Scrum is on product delivery level, so the position you are in, the environment, organisation and geographical position could be all factors that have an influence on your choice.
Although SCM is very popular nowadays I believe that quite soon there won't be as many Scrum practitioners as today - organisations are changing from Scrum to Kanban or Scrumban or Scrumbut -, so it's value will decrease, on the other hand, there always will be a need for project managers, so the value of prince2 will be the same
If you are looking to the future Prince2 may be more portable if you leave an agile environment. In North America Prince2 is more government work, but even there jrosell is right that PMP is much more common.
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.
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.
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