Good question, but I beg to differ:

1. The definition of platform is not clear - is platform a version of SQL Server or a version of the hardware or OS?

2. If we say that platform is a version of SQL Server, then the correct answer is that CONVERT has more options when compared to CAST

3. CAST is, internally implemented as CONVERT - Refer my post on the topic here: http://beyondrelational.com/blogs/nakul/archive/2011/07/18/cast-v-s-convert-is-there-a-difference-as-far-as-sql-server-is-concerned-which-is-better.aspx

The question leaves a lot of room for interpretation errors. The answer does not clear out those points either.