I've always found the comparison of these two films strange.
One is a great crime drama and the other is a great fairy tale.
Slumdog is probably my favorite fairy tale film. The soundtrack is beautiful, especially Latika's Theme. And the film itself resonates with me because I saw it without knowing what it was during a stressful period where a surprise fairy tale legitimately put a smile on my face. I can never discount visceral reactions to art.
City of God resonates with me for entirely different reasons, but none that makes me appreciate Slumdog any less.