If you are thinking of getting an ereader to do a lot of reading, you are overlooking the advantage of Kindle/Nook over iPad. The iPad has an LCD screen and is similar to reading a computer screen. The ereaders such as Kindle use e-ink and have no backlight. The reading experience is similar to a book, not a computer screen.
If you are looking to maximize functionality, clearly the iPad wins over any single-use device.