Kobo, with qualifications.
For a simple reason, Amazon locks you in to their ecosystem for purchase. With the Kobo reader, you can purchase books from Kobo, BooksonBoard, Borders, Sony and any other ebook retailer that provides downloads in epub format. With a Kindle, you are limited to Amazon or a small number of ebook sellers that still suppoer mobi format. Kobo (and most epub readers) also support library borrowing. Kindle doesn't.
Unless ... you don't mind stripping DRM from ebooks. If you don't mind from a values standpoint, it is fairly easy to set up automated DRM stripping ... but that depends on whether your wife is comfortable with taking a couple of extra steps to sideload content or whether you're prepared to always load her books for her. In that case, I would probably go with the Kindle in that it's a faster reader, has the best dictionary functionality of any ebook reader, and has notes/highlights/jump to page features that aren't present on the Kobo.