Man, what's up with all the hate going on in this topic? Both games have top-tier musical scores, and which is best is a matter of stylistic preference. Personally, I'm not a huge fan of the vocally-driven Nier OST, but I can still appreciate its music as pretty damn stunning.
That said, this seems like a pretty good opportunity to plug Xenoblade, which has music that rivals Nier's and CC's:
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http://www.youtube.com/watch?v=SBpfwpOPXNc&NR=1&feature=fvwp
I watch Planet Chasers Starlight Excellent all day, every day.