Think this has gotten way of topic.......especially for jbcohen original topic. Came across this link which i found interesting. I know the Sony PRS -505 has being replaced but still many around to be purchased 2nd hand as I have. The author of this piece also agree same as me that Sony is heavier than the Kindle. I am sure newer Sony's would be lighter but I can only speak of my own experience of using the Kindle wifi and Sony PRS 505. . Here is the article...
http://tinyurl.com/6gq3wqv I know its a bit outdated but some of the pros and con are still valid.