Pretty much anything is better than the Nook.
If your wife has an irrational bias against the Kindle, double-check that she doesn't have an irrational bias against the Kobo as well! (Before you buy it!)
The main differences as I understand it, between the Kindle and Kobo, are:
Kindle has amazing ecosystem (Amazon is usually cheaper, has more books if you count indies)
Amazon's customer service, should you need it, wins hands down
Kindle has WhisperSync for personal documents
Kindle has direct integration with OverDrive library books
Kobo has far better typography, if you happen to care in the slightest about that sort of thing
Kobo collections can be managed via calibre without first jailbreaking the device
Kobo can checkout library ebooks even if you don't live in the U.S.