If you're looking for a NAS with a lot of features, then Synology is the brand for you. The new DSM 4.0 makes their NAS devices even more interesting, and the performance is more than acceptable as well. If you compare the price difference between the DS212j and the DS212, then the performance difference is comparatively small. The DS212j costs an average of £166 and the DS212 an average of £238, which is a big price gap not justified by the performance. The selling points of the DS212 are therefore its memory card reader, a USB connection on the front, USB 3.0, and a more luxurious case. If these features aren't important to you, then you should opt for the DS212j.
The RS212 is a special NAS device aimed squarely at the business segment, although we won't be suprised if several Hardware.Info readers have a 19-inch rack at home. The sturdy metal case does cost a bit more, an average of £160 more than the DS212 that has about equal performance. We can imagine that the price won't pose a problem for business users that will enjoy the added security of a second network port and the extra year of warranty, and especially the option to place it in a rack. One downside is that its power supply isn't very energy efficient.
Generally speaking, all three of these are interesting NAS devices. The DS212j is a fast and versatile entry-level NAS. The DS212 is worth the extra expenditure if you need extra and faster connections. Both the DS212j and the DS212 earn the Hardware.Info Silver Award. The RS212 is only interesting if you have a rack in which to mount the device, and has a relatively high energy consumption.