I dont know anything about the sony one but I have a kindle and LOVE it - cant imagine life without it now and the customer services are brilliant if you need them (I had a minor query)
Kindle is fabulous. It's not back lit so it's much easier to read, the screen look like the page of a book. You can download books straight from the internet, categorize them...loads. Would highly recommend it.
Project Gutenburg is filled with free books, most are classics but there's still a huge selection. Ebooks3000 is another I use sometimes and you can search Amazon by lowest price first, they have offers on all the time, mostly free old books but a lot of recent ones too.
I love my Kindle-currently reading the Millenium trilogy by Stieg Larson-for the three books in total it only cost me £7.98! Was dead against the idea of an e-reader at first, then hubby got me one as a suprise and couldn't be without it now!
I have a Sony touch and I love it, its fantastic Definitely recommend the touchscreen option. I do loads of travelling and I have around 120 books (mostly free from Project Gutenberg as already mentioned) which is wonderful for long haul flights and even just putting in my bag and taking out - I love to read anywhere. I haven't got my hands on a kindle yet though so I can't really compare. I like the fact that Sony has lots of available formats, but the kindle could be the same, I don't know!
why not look for an internet tablet with android, and get the kindle app (free) then its not just an ebook reader but also a web surfer and media player they start from around £100. I use my phone (android) and my ipod touch (with their own ebook app and the kindle app).
Also if you go for kindle, you can read on your pc too
My father bought my DD (who is 12) a Kindle for Christmas. I am using it, too -- and never thought I'd like something like this but I do. She likes it, too.
I adore my kindle and couldn't be without it. There are also pirate sites were you can download books and use Calibre to change them over - not that I would do that!
amazon do have free books too - worth keeping your eyes on the site as some change from time to time - ive picked up some good book by being on line at the right time