What's the better preposition to use with "love" -- "love for" or "love of"?

It's not an either-or situation. You can use either preposition.

Love of money is the root of all evil.

The Love for Three Oranges (opera by S. Prokofiev)

I have nothing but love for my children.

"For the love of God, why would you say such a thing!"

Note: "of" may be used to express that the object of the preposition is the thing that loves, as in the last example above. You could say something like "I would do anything for the love of a good woman," meaning that you want to get her love (as well as, presumably, to give it).

I think, "love of" is ambiguous. Consider this. "The love of his wife was undiminished." Who has love for whom? "The love for his wife was undiminished." It is clear that either the speaker or someone else loved his wife.