PC Pro magazine have a full report on ebook readers this month, Kindle comes out top again.
Before jumping for either reader you need to consider the price of books for the device.
For the Kindle you get books mainly from Amazon, and they're usually pretty cheap, the price of Kindle versions is listed on the main page for books. you can also buy books from other sources as long as they're drm-free epub or Mobi-pocket versions.
The WH Smith Kobo reader can use books from a number of stores, but not Amazon. I've not checked the prices for epub format books, but when I had a Sony reader for a while last year I found that the books were invariably more expensive from WH Smith & Waterstones than from Amazon.
We're at a position with ebooks very similar to that in the digital audio market 5 or so years ago, when iTunes only catered for Apple's own AAC format, so only iPod owners could use the biggest store - everyone else would just rip their own CDs or use illegal download sites...