What E-Book device to buy is a matter of personal preference. Taking the time to read up on what devices that are available and the functionality they provide is worth it. There are some links that are essential in that regard.
Mobileread also have a wiki containing information about E-Book devices. It has a couple of pages containing technical specifications on the different devices which have been very useful to me.
These links and spending time reading up should be enough to be able to decide what you would like to have.