Definitely the Amazon Kindle, it is famous for providing much better content support compared to other ebook readers. The ebooks on Amazon Kindle is also cheaper than that on Sony Reader.

I love my Kindle 2 very much. The Amazon Kindle is better than Sony Reader also because it has free wireless and text-to-speech function. There are some good comparison between them here:

http://www.speedreaderxreview.com/kindle-2-0-vs-sony-reader-prs-700bc-comparison

http://www.amazon.com/gp/redirect.html?ie=UTF8&location=http%3A%2F%2Fwww.amazon.com%2Freview%2FR3ULAFVHBGYVQP%3Fie%3DUTF8%26ref%255F%3Dcm%255Fcr%255Frdp%255Fperm&tag=moreinfo-po-20&linkCode=ur2&camp=1789&creative=390957

And in case you don’t know, Amazon has recently put a price slash on the Kindle 2, and it adds in international wireless access and native PDF support… So take the chance if you want it.

Hope it helps