Welcome! I can warmly recommend the Sony readers, having owned one for 2 years. However, the Kindle is a great device as well, and has a lot of users who love it. The new Sony models (the 5" PRS-350, 6" PRS-650 and the 7 " PRS-950) and the Kindle 3 are the only devices currently using the new Pearl e-ink screen with better contrast, so I would suggest taking a look at those. Try looking at video reviews from this site:
http://www.the-ebook-reader.com/
as well as checking them out in person at a local Best Buy, if you can, to get a feel for what is good for you. If you do plan on using the library a lot to read e-books, the Sony models would probably be a better fit, however, as not many libraries carry books that can be read natively on the Kindle.