I know nurses who have taken both the AMSN and the ANCC med-surg certification exams. From what I've heard, the AMSN focuses more on body systems and disease processes, while the ANCC has questions related to delegation, collaboration, ethics, patient management, etc. in addition to disease questions. There is a *huge* body of potential questions that your exam can come from though, so results may vary.
I personally took the ANCC exam from a purely financial perspective. My employer is part of the Success Pays program with ANCC, where we get two attempts, and only pay for the exam once we pass. Like Sammi, my employer gives us a raise for being certified (3.5%!) which is nothing to sneeze at, and it also ties into our clinical ladder program.
For me at least, the actual studying for the exam reinforced my knowledge base more than the exam itself. I do feel like studying was an excellent refresher, and the CE requirements for certification renewal force me to continue to stay up to date with articles, classes, etc.
Hope some of this rambling helps you, and best of luck! :)