I suspect the issue is with the Kindle's design. I had two cracked screens in the course of a year with a Kindle 2i, yet my Kobo Touch has held up with a year and a half of use. This in spite of the fact that the Kindle was always in a case and the Kobo was not.

After cracking open the cases of both, the reason seems clear: the Kobo has a rigid frame while the Kindle had a more flexible support structure. The latter seems like a poor design decision for a screen with a thin glass substraight.