I personally found FFVI's translation to be a bit more... well... 'mature' than FFIX's. I mean, the dialogue in FFIX had a tendency to be somewhat childish, whereas in FFVI the script was often a little more meaningful. So in my opinion, FFVI has this one edge over FFIX, which is strange, considering Alexander Smith's fine work with FFVIII, and in particular, the beautiful dialogue in Vagrant Story.

Perhaps Square wanted FFIX to be slightly less grown-up than its predecessors?