With Google Sync, the servers have access to your plaintext data. This means that Google Inc, the engineers at Google with access to the servers, any attackers that compromise Google's sync servers, or any governments that compel Google to provide them access to the servers, all have access to everything (photos, contacts, calendar entries, documents, etc.) that you sync.

With Flock, the data is encrypted with a key that only the user knows before being transmitted to the server. This means that nobody with access to the Flock servers can see your data.