Is cement board a better soundproofing material than 5/8 drywall?
The less dense material would be better for soundproofing, the 5/8" drywall.

There's a lot that goes into soundproofing and there's even professions within the construction industry that specialize in it, acousticians. That being said I did a bit of research with this in college and it turns out that less dense materials provide better soundproofing. The more air bubbles in the material, the more the sound bounces around before reaching the other side of the material.