I would go with the Personal Documents method as you will get syncing of farthest page read, highlights, bookmarks, etc. If you want to sideload books to an iPhone you will need to use an application like DiskAid on your PC to be able to view and copy to the correct iPhone folder. Its a PITA and quite frankly it didn't always work right in my case. Plus sideloaded books only synced farthest page read, they did not sync highlights and bookmarks.