NPD's more accurate, but VGC gives you 100x the data and cool online tools to analyze it. From my perspective, it's like VGC is the author and NPD is the fact checker that pops in to let VGC know when an important detail is slightly off.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.