I'd guess more often then not NPD will get it closer more often then VGcartz, but do to the nature of the buisness, this isn't going to be the fact 100% of the time... as it seems likely the case recently do to the released numbers like Ioi said. Later on, it's likely they'll be more accurate.. it really depends on the formula they are using.
That's the trick really. You can do this, have a much larger base, and still be less accurate depending on what kind of equation your using. The problem places like NPD have is that they are paid for their sources... so they don 't want to tinker too much with the formula because if they go too far one way and way over shoot they're going to lose a lot of buisness.
If NPD and an independent site that wasn't free both had 60% of the market... i'd bet on the independent site being more accurate because they would be more willing to tinker with their formula to get it closer, instead of saying something is "close enough."
