Does the Dell solution do just iSCSI or does it do NFS as well?
What kind of raid do you use in your synologies, raid 5 or 10?
Have you tried the raid 6 option?
With raid 6 you can loose 2 disks without loosing data.
I don't have a synology. I have a qnap though, but I might be interesting in replacing that with a synology
Only using it for backup. It is a two disk model with raid mirror so there is no comparison to your synology.
I have an Omnios box with ZFS for production storage.