Canalys' numbers are only about the second quarter, Nielsen's survey is about the first AND second quarter.
Canalys' numbers are more accurate, because they are about only the second quarter and they include iPhone 4 sales.
The percentages are (of course) the average over the respective periods of time.
Android sales have been climbing fast, that means the longer the time period, the lower the percentage.
That's why Canalys' numbers give a more accurate picture of how the sales numbers stack up today.
Nielsen has some other interesting numbers about usage share and satisfaction rates.
The two survey combined can give you an idea of how fast Android market share is growing (if you do some maths):
The average Android market share over six months is 27%, take only the last three months and it's already at 34%.
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