BlueLegend64,
The E17 has a built in Digital-to-Analogue-Converter (DAC), so it will act like a quality stereo sound card when you plug it into your computer. The E17 also allows treble and bass adjustment on the device. It would be classified as a portable DAC + amp device.
The E12 (Mont Blanc) is a different kind of product, it is a portable amp that is between 3 and 4 times as powerful as the E17's amp. The Mont Blanc is intended to be plugged into another source like the headphone out of an iPod, computer, etc. I am going to use mine with my iPod and my gaming setup, as the last component before I plug in my headphones (AKG Q701, which benefit from more powerful amps than a headphone designed to work with an iPod).
If I needed one device to carry around with a laptop, I would buy an E17. It's a pretty good all-in-one device. But, since I have a nice soundcard in my desktop computer (my laptop is broken), and a separate DAC for other devices, I choose to get an E12 Mont Blanc so I would have a really powerful amp that I can easily take and connect to multiple devices around my house and (hopefully) at work.