Hi Dave,
What are your Recovery Points Objectives (RPO) and Recovery Time Objectives (RTO)?
RPO is how much data are you will to lose during recovery of the affected volume, LUN or database? This determines how frequently how run SMVI and how frequently you replicate data to another storage device.
RTO is how long can you afford to be down while the recovery process takes place? If you can only tolerate a few hours at most, then keep most of your backups on your FAS storage. If you can afford to pull an all-nighter to recovery the data, then move it to the Data Domain systems.
Another factor to consider is, what kind of disasters do you plan to be able to recover from? If you're only looking to protect from user errors, software bugs, etc., then keep your backups on the FAS. If you want to survive damage to your data center then you want your backups on the remote Data Domains.
Michael