Folks at NokNok.tv has now posted an imaging comparison between Lumia 620 and Lumia 520. Lumia 620 has certainly upper hand with Flash which is absent on Lumia 520 and in past has proven that it can take good low-light photos as well. But this comparison shows that other than conditions when lighting is very bad, Lumia 520 has as much of capable camera as on Lumia 620 and the cameras on both are really good for the price range in which these two fall. Check some of the comparison shots below.
Low-light without flash: