For me, the killer feature of Obsidian Sync is that it makes version control available at a right click of the document. It works in a mixed environment and, I think, I read somewhere that it will be required for mobile.
For now, my hack with keeping the Vault on Dropbox on a single machine, to sync it with 1Writer has not caused any problems. However, I don’t need Dropbox for anything else and would rather not have it running just for this task.
My desktop is a Windows machine and iCloud is not really an option.
Finally, the early bird pricing is a good argument - and it supports the developers …