From what ive heard:
Internal - if something breaks it is a huge PITA to repair or replace. Take that as a grain of salt as I have never dealt with internal. Much more cleaner less lines - less of a snag hazard.

External - I have a NON cascading 8:1 that could be converted to a cascading 16:1. I have not needed to convert to anything more than what I use, which is the 8:1