With an iPad you can do most things that a regular computer will. There are hundreds of thousands of apps for the iPad. There's a strong rumor that Microsoft will be releasing "Office" apps - "Word, Excel etc. "
The Kindle Fire will do no where near as much as the iPad. The Fire is designed to consume products from Amazon, for example you cannot put your own videos on it.
Here is a link to one of many articles defining the issues users are finding with the Kindle.
http://www.useit.com/alertbox/kindle...usability.html