I've played 2 FF games, FFVII and FFIV DS. Before I start ranting let me just say that both games are amazing, so although I might be picking on them, I get nit picky with all games.
Difficulty: FFVII wins
whenever I talk about games in general, the difficulty is big for me, FFVII is challenging without being merciless, while FFIV on the other hand is about as easy as piloting a supersonic jet through a tornado! There were several occasions where I had to fight two insanely difficult bosses right in a row with no break in between.
Combat: Tie
The one thing VII has over IV is that the ATB is quicker while still making it feel that it's not instantaneous, but I felt with 3 characters in battle I found myself trying to do too much at once, while IV's ATB's were much slower, the 5 character limit was more balanced
Music: Tie
there's not really much to say, Uematsu is a great composer
Characters: FFVII wins
I felt that each character in VII was more vibrant and colorful than those in IV, most people don't know that you could go the entire length of VII
without ever meeting Vincent or Yuffie although that's another point, some of IV's characters felt uneeded or too stereotypical such as Edward
who doesn't stay in your party for very long, or Rosa who always had to be by Cecil's side