i think the main difference is that the MS Sync Framework is a much broader, thus more complex to use API. OpenAccess support for disconnected clients however uses only a few additional classes focusing on synchronizing offline changes to the connected world with the main API class being the ObjectContainer. This container implements a common interface with the main connected API, the ObjectScope. Therefore the gap between online and offline API is minimal. Also, the efforts needed to push changed, added and removed objects back into the connected world is minimized for you.
I also think, our API is more appropriate for limited environments as there is no dependency on file I/O.
the Telerik team