Heroes III or Heroes IV (of Might and Magic): Which is better?

I was about to give Heroes IV another try for old times' sake, until many in the net argued that Heroes III is better. I have both games, so I’m confused which of the games I should install and play. Which of the two do you think is better (gameplay-wise)?

And don’t suggest Heroes V. I have that game, too, but I find the two games better than V.