Your point was that steamspy collects more data/numbers. But actually SuperData cover a wider market and say they collect more.
As to whether either are accurate is up for debate. But they both use hard data in most of their calculations.
What kind of data they collect to know digital sales of a game? Hardware makers and publishers are not giving information for them, so I don't understand how you can debate accuracy against numbers based on Steam Web API.
Superdata try to cover all market but I can't see how they may have a minimal accuracy. Not even big companies like NPD and GFK tracks digital sales of each game.