* Chunked uploads. Try to modify a 1GB file in OneDrive. The complete file will be resynchronized. Dropbox just uploads the chunks that have changed.
* LAN sync. If you are sharing files with family/colleagues that are on the same network, Dropbox shares file chunks peer to peer. This usually results in much faster synchronisation than downloading from the cloud.
* Dropbox has file requests. People can upload files to your Dropbox, but what others upload is not visible.
* In my experience, the Dropbox client is a lot more stable than the OneDrive client on OS X. OneDrive crashes or sometimes cannot sync certain files.
To me the speediness, file requests, and more reliability is well worth whatever the price delta is.
(Note: I have both, but I only use OneDrive as an endpoint for Arq backups.)