Well, some people actually think they are the same diety. There was even a Toltec ruler named Quetzalcoatl.

As for the similarities, they both where feathered serpent gods that had both anthropomorphic and animal forms. Both were part of creation triads and were associated with agriculture/fertility and governing civilization. As part of their myths, it was foretold that they both would return and restore their respective cultures. White hair was a part of their human depictions.

As a side note, when the Spanish invaded Mexico, it is alleged that the native peoples believed that they were these dieties. However, there is a lot of dispute over this claim these days.
Source(s):
http://www.associatedcontent.com/article/217747/the_mayan_mystery_who_was_k... | http://en.wikipedia.org/wiki/Quetzalcoatl | http://en.wikipedia.org/wiki/Kukulcan

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