I've had a ReadyNAS Duo since 2011.
The software is starting to feel a bit old, but the unit itself is working, as has served the house for 5 years now.
The only mechanical thing is the fan - it's become very noisy, I need to get a replacement.
The unit is configured with 2 x 2TB disks in RAID1 (mirror).
I have been expecting the day one of the two disks died for some time.
Coincidentally, that day was two weeks ago.
Had a spare 2TB bought for this very reason lying around, popped the old one out, and the new one in, and after 9 hours, it's fully rebuilt.
Although a 4-drive will give higher read data-rates, it's unlikely that I will need that.
The NAS is used for all photos, audio-files, and some video backup.
TV stuff is kept at the receivers for now; nothing's that important if it came over the air...
If you need a or b is probably more of a personal taste.
The ReadyNAS was recommended by a friend, and it has done what it promised so far.
Again, as with most other things, you mostly get what you pay for.