You can see the section "Media Formats Supported" on the official page here for the ebooks that Sony Reader supports:

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10551&storeId=10151&langId=-1&productId=8198552921665921180#specifications

Not sure if it helps, but I love my Kindle 2 very much. I’ll say the Amazon Kindle is better than Sony Reader because it has free wireless and text-to-speech function. There are more ebooks you can find available for the Kindle, and books on the Kindle are quite a lot cheaper too. 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

For the file formats supported by the Amazon Kindle, you can see "Files Kindle Recognizes" section on the official page here:

http://www.amazon.com/gp/redirect.html?ie=UTF8&location=http%3A%2F%2Fwww.amazon.com%2Fgp%2Fhelp%2Fcustomer%2Fdisplay.html%3Fie%3DUTF8%26nodeId%3D200375630&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