What is the difference between the 'e' models and non 'e' models?
For the Disk Station Series, refer to the following image,
As you an see, the DS-106 contains three more functions used for the business enviroment, which are Secure FTP, Secure Network Backup, and Windows ADS Support. And the DS-106 also has twice as much memory with a wider bus vs the DS-106e, which gives it better performance for where there are multiple users connecting to the server at the same time.
For the Cube Station Series, refer to the following image,
Functionality wise, both CS-406 and CS-406e do the exact same thing, the only difference between the two units is the performance. As you can see on the graph, the CS-406 has better performance where there are nine users connected to the server, achieving 52.89MBits/Sec in RAID 5 Mode, while the CS-406e only achieves 39.74MBits/Sec in RAID 5 Mode. So, the CS-406 is better for heavy multi-user enviroments, such as the SMB/SME environment, while the CS-406e is great for SOHO use or even Home use (as a media server!) where there is a lot less users requiring to connect to the Server.