Hello there! Welcome to MobileRead.
I'm a very happy Kindle owner and user, but I do recognize that I could have been happy with the Sony if I had seen it first. One advantage to the Kindle is the fairly quick availability of new releases in Kindle format at a maximum price of $9.99. But if you're looking at SF from Baen and all the free books from here, Feedbooks, and ManyBooks, either device will work.
Both the Kindle and the Sony PRS are challenged when it comes to usefulness in displaying PDF files -- particularly those with complex graphics or tables.
Amazon offers daily delivery of newspapers to the Kindle over their Whispernet service, which you can't get directly from Sony or any other seller of the Sony PRS. But, with only a small effort, you should be able to use calibre to download the articles from online new sites and then send them to the Sony reader via your PC. (Or so I'm told.)
You should definitely talk with some of the Sony users here at MobileRead and some of the Kindle users -- and then make up your mind based on the ways you think you'll wind up using an eBook reader.