Best folder sync software

I have an iSCSI device attached to my Mac (two actually). On drive one I have several hundred gigabytes of data. I would like it on both iSCSI devices so I downloaded Sync Folder Pro. What I expected was that after the first pass, all other passes would be the difference, but this is not the case. It appears Sync Folder Pro is doing a full sync each pass. Well, this takes forever to sync that much data...

So, on to my question...who makes the best sync folder/file program for Mac?