@ScottPSilver welcome to the Rockstor community.
I’d be remiss not to mention @GeoffA’s mantra of having good backups, as a NAS is (by itself) not a backup system (but of course that’s a budget/risk thing, too). So, aside from all of the above, consider to also have another backup of what you have on Rockstor, so you don’t end up with …
With that out of the way …
I personally would go with the larger “renewed” version of your HDD. Part of the risk offset is supposedly the fact that one has RAIDs set up, but then also see comment above re: backups 
There was an interesting discussion around RAID levels a few years ago you might be interested in reading. Some things have changed in the meantime obviously:
rebalancing is still not substantially faster on RAID5/6 as far as I know, though the btrfs devs keep working on it.