Thanks. Unfortunately, I don't know how to write .NET. I did find a workaround for now. I run the preview every time but then before the second run without -preview, I do a FINDSTR in the log and if it finds the phrase "Nothing to synchronize" then the script exits at that point. If it doesn't find this string, it proceeds to run the script again without -preview.

You can use .NET assembly and generate the list based on results of Session.SynchronizeDirecotries:
https://winscp.net/eng/docs/library_session_synchronizedirectories
https://winscp.net/eng/docs/library_synchronizationresult

Is it possible to have the log write the output of the -preview switch while actually synchronizing?

I'm currently running my script twice. First pass, it uses -preview so I have a log of the "Differences Found:". I really wish I could run my actual sync and have this nice neat list of the differences found all in one pass.