Unity iOS Xcode Build: Replace or Append?

It is my understanding that Append will sort of just replace the parts of the project that have changed, while Replace will clear the target folder completely and build all the files from scratch.

Is one better than the other? If I choose Append, do I run the risk of including "old" / "cached" files in the build (such as images)? Is it better to choose Replace to make sure that all files are "fresh"?