I recently went through the same thing trying to decide between both of these over the past few weeks, and my advice would be to get the DS213J. With the exception of running all kinds of apps (way more than downloads, media, and backup stuff), which I don't think either of us will be doing, the only time you're going to notice a big difference is during the original disk verification process. Even then each 4TB disk will still probably take eight hours vs. ten hours, but this is a once and done thing. The only exception to this would be if you needed to rebuild after a disk failure or want to swap drives, but you can do this overnight while you're sleeping, meaning the dual core is not worth the extra $130.
Also, the DS213J's fan will turn off if the disks are hibernating and its running cool enough due to the fact that Armada 370 runs very cool. The same thing isn't true about Armada XP (DS214 & DS214+) and most previous models including DS213. Originally, DS213 and other previous models did the same thing, but Synology has since changed the fan profiles starting with DSM 4.3.
FWIW, the fans are not loud, but I'm a silence addict, so keep that in mind. It's also nice knowing that the DS213J uses only 3.65W while the disks are hibernating, because no other unit comes anywhere close to this.
In case you're interested, I'm personally not using Raid 1 for my setup. I have two basic 4TB volumes and I have backups scheduled for "important things" like documents, etc., cloned from volume A to volume B. I also do two external backups via usb (one off-site/one on-site). Some of the advantages to not using raid 1 would include obviously more space, retaining a duplicate of a file should you accidentally delete something, time backups allowing you to have multiple revisions of file(s), and if your NAS gets damaged, you can simply pull the drives, put them into external enclosure, and plug into a computer running Linux to get your data. I believe the same thing can be done with raid 1 disks, but it's definitely not as simple.
The only possible advantage I see to raid 1 would be rebuild times after a you get your disk back from an RMA due to a disk failure. Personally, I don't feel the time saved is worth the space lost.
Lastly, I'm not sure which drives you plan to use, but if you decide to use WD Reds in a non raid environment similar to mine, don't believe anything you read about TLER being bad in a non raid environment.
Disclaimer - It's late, I can't get to sleep and I'm swyping all of this via Tapatalk on my LG G2 lol
Edit - I should add that every computer owned has nothing but SSDs (Plextor or Samsung) and not a single mechanical drive internally, meaning I'm used to everything being quick. Just wanted to put my DS213J recommendation into perspective
