The RN316 is the model that replaced the Ultra 6 in our lineup.
I'd probably be looking at that or the new RN424 (if only need 4-bay, but since you want dual-redundancy which I recommend you'd get 50% more volume capacity using a 6-bay or double the capacity using an 8-bay, so I think a 6-bay or 8-bay would be the sweet spot). Don't forget there's also the RN526X. The RN626X is our high-end 6-bay desktop. Unless you're looking to transcode 4K or do something else that requires a lot of CPU then in the home the RN526X would be more than powerful enough. The RN526X has two 10G ports as well. Note 10G ports are backwards compatible with gigabit but not 100Mbit. So if you have a router with 100Mbit ports only you'd need a gigabit switch in-between. The RN626X has two gigabit ports in addition to the 10G ports.
The RN526X would give you a big step up in performance over the Ultra 6. The RN316 would be an improvement, but not as noticeable.
You can buy diskless units or units with disks installed.
The number of bays is an interesting trade-off now, since disk capacity has grown so much over the past couple of years. I do like to start wtih at least one slot free for future expansion - and doing that with RAID-6 requires at least 6 bays. Though if you are using the ultra as a backup, I'd go with RAID-5.
The RN42x and RN52x are both solid choices - I use an RN524x and an RN526x myself, and am quite happy with them. The 10G network connection is useful for enterprises where there are lots of users, but of course it does work over gigabit.
The RN21x is about the same speed as an ultra and might be enough. It has an ARM processor, which does put some limitations on the apps you can run. If you want the biggest range of apps (not something I care about myself), then x86 NAS are better choices. And your desire for RAID-6 does push me towards a higher-end NAS.
Plex transcoding actually works better on the RN21x series than the RN31x - because the RN21x has 4 cores, but the RN31x only has 2. The RN52x can trancode 1080p in real-time, it can't handle 4K.
Depending on your budget, I'd go for an RN424 RAID5, RN524X RAID5 or RN526X RAID6. If you have two NASes, maybe RAID6 only on the backup RNDU6000 and RAID5 on the main is enough.
I didn't understand what you meant by WD Red 5400RPM and WD Red 7200RPM, unless you're talking about WD Red Pro 7200RPM, afaik all WD Red are 5400RPM (IntelliWrite).
I didn't understand what you meant by WD Red 5400RPM and WD Red 7200RPM, unless you're talking about WD Red Pro 7200RPM, afaik all WD Red are 5400RPM (IntelliWrite).
Correct. The Red Pros have enterprise performance specs and run at 7200 rpm. The Reds have consumer performance specs and run at 5400 rpm.
I use the Reds myself - they run cooler, and I am willing to give up a bit of performance get that. In most cases my RN520 NAS are limited by the gigabit network, and not by the WD Red performance.
If you can swing it, I would highly recommend the 528x.
If you need to save money, just keep it to 6 disks, then you can expand later as you need.
Or the 628x if you got the money and expect to do alot of transcoding with plex or something similar.
edit, also once you do get a new device, I would suggest you look in to upgrading your ultra6 to os6 (this requires a factory default, so you would need to get all your data up and running on the new device first).
Assuming you get a new os6 readynas to use as your new main, after upgrading to os6 on the u6, you could setup regular backup jobs or even use replicate for remote backups.
