My advice, as owner of both a Nook and a Kindle, would be to go with a Kindle or a Sony. Kindle works great, it's soon to get an
update to make it even better, and I've heard wonderful things from folks who swear by their Sony readers.
PS: The PDF pan and zoom function coming with the next Kindle update (May 2010) should make reading PDF files a breeze! Great news for aged eyes! I may just buy a few graphic novels to celebrate!