I agree with Chocolate, となる does sound more literary/formal than になる Additionally, in a thread on Yahoo 知恵袋 (http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1113732101), namaste981712 brings up a few good points about his/her ideas about the difference between the two. The following is a translation/summary of the main point: Both 「と」 and 「に」 are used to express changes in actions and situations, but 「と」 focuses more on the result, whereas 「に」 focuses more on the process of change itself. That said, there are situations when using 「と」 might sound unnatural. For example, the expression 「大人となる」 (to become an adult) is a bit odd. Since growth comes gradually as a process of change, 「大人になる」 sounds more natural. In the case of 「成長して彼は立派な大人となった」 (He matured into a fine adult), it sounds pretty natural because 「立派な大人」 (a fine adult) can be thought of as the end result which is the main focus of the expression. Another example: One can say 「星になりたい」 (I want to become a star), but saying 「星となりたい」 sounds unnatural. This phrase expresses a mere wish (a wish that will never be realized, because a person cannot physically morph into a star, it's impossible) - therefore, using 「と」, which focuses more on the end result, somehow feels unnatural.

http://my.opera.com/japanesewithmie/blog/show.dml/3767424 This page states that「となる」normally doesn't follow na/i- adjectives.

となる sounds to me more literary than になる