What is advantage in open or closed shoe

The 15-195B has an open cam designed to work with specific pivot bars. There isn't an advantage of one over the other. It's more a matter of, you either have an open cam shoe or you don't. 

You just want to replace what you have. The alternate pivot shoes in this style are not upgrades. They're just different styles meant for different tasks.

Hopefully that clears things up. Let me know if you have any follow-up questions.