For Mike_D, the feature where you can hook up additional units, really isn't as important in "home" use vs a business. When you consider how long it will take you to fill the unit up, by the time comes to get an expansion unit, there will be a replacement.
The replacement will be either a newer faster higher capacity "base" unit or the expansion unit may not be in production anymore and the replacement may not be compatible.
HOWEVER, that means you can buy the expansion unit on Ebay for a fraction of its initial value.
What I do is as I need more space, I buy a newer faster bigger NAS and rotate the old NAS into a backup for it. The oldest smallest backups eventually get sold.
Having a smaller slower (or for that matter say 2 units) for backup doesn't really matter since generally you only add some data onto the backup and it would be rare that you need to restore an entire NAS.
The one thing I've learned about them (I use Thecus since that is what I happened to buy when I got to serious units), is that the more you spend, the faster they get. When I started with WD Sharespace cubes of 4TB, they were maybe 20MB/sec transfer at best. So try and load Digital Photo Professional with 1000 RAW images. It takes literally all day. The Thecus 8900 I'm running now moves data at about 120MB/sec and its darn fast. At the time I bought them the WD was about $700 a few years back. The Thecus is about $2500 plus the cost of 8 drives (say $4000 total). But if you really want to work with it, you need to pay to play.
Buy the unit with the fastest processor you can. Processor power is more important than the amount of memory in the unit. Plus check out speed tests on the web comparing units.
Hard drive speed makes a real difference too. You need to look at recommended hard drives too. NOT ALL VERSIONS of drives work the same way. There are ones that work with RAIDS and some that don't. So a WD20EADS drive may not work while a WD20EARS drive does (interestingly WD recently recommended that ALL their GREEN drives not be used in RAIDS since they may not hold up under the duty cycle) I disagree with the WD recomendation since Thecus tested the drives to see which one WORKS in a raid. As far as duty cycle is concerned, with 1Million hours MTBF, hard to see the drives failing under "home" use (results may be different in a business where the drives are running and accessing data constantly)...
A work around for the speed is to copy the files you are working with to your computer and keep a "backup" on the NAS. You get the speed and as you copy the files over, you can take a small break and get some coffee (or other beverage).....
Last, I use ALLWAYS SYNC to synch the NAS and the backups. Work great and the basic version is free (of course if you use it a lot, the software suggests you buy the PRO version, which is well worth it).
Too Much Camera Stuff......