Can I use built-in software from either box to make them mirror each other?
Yes. I use rsync backup jobs for each share myself to back up my pro-6 to my RN102 and RN202.
After you get the RN316 running as your main NAS, you can also convert your ultra-6 to run OS 6. After that completes, you could could use the new ReadyDR feature (which backs up OS6 snapshots as well as the main shares).
Note that OS-6 on the ultra is not supported by Netgear, but there is information here on how to convert (and many people here have done it).
