The back of the systems are different as the Original Kindle users have stated that it can come off to easily when dealing with.
The Kindle 2 looks and feels more modern-day and locks in location in such a way that its not going to come off unless you desire it too.
There are likewise no "usage" charges for downloads and therefore no limitation to go beyond.
You can fill as much as your Kindle will keep.
For me this was perfect considering that we are set up on our own protected wireless network in your home.
If you do not have Wi-Fi access I confess this might be a draw-back for you.