I have used a TabletPC (colour LCD screen), Nook Wi-Fi and Kindle 3 Wi-Fi.
Of the three, I would pick the Kindle 3 for reading ebooks. It has a really good battery life, good contrast for the eink display and page turning is very quick. The Nook was nice, (and the touch screen is useful for the included Sudoku and Chess games), but the battery life was lower than what I see for the Kindle 3. The page refresh time was also higher, though the new firmware might have changed that (I haven't used it after the firmware update).
One reason you might want to go with the Nook is if you have a lot of books in epub format. But if you use the Calibre software suite, converting books to the Kindle format is very simple. (Can convert from pdf, txt, html, lit, epub, mobe, etc) Drag a book into the software, right click on it and send to device in desired format. (Think of it like using iTunes with an iPod/iPhone)
The only thing I read on my TabletPC is comics. I cannot see my self going back to an LCD screen for reading a lot.