I think Koichi Sugiyama is the better composer. As he influenced them both and produces, in my opinion, the best music for a major RPG series in Japan.
However, just focusing on the two choices for the thread, I couldn't really choose. My default choice would have to say Mitsuda for ultimate enjoyment of his music, but Uematsu has produced far more music. I suppose I have to say Mitsuda however, for his consistency and more awesome work.