Didn't try AltWinDirStat, but did try FastWinDirStat.
The thing is, FastWinDirStat uses a licensed propietary component. No problem for me, but the author did have some back and forth with another user on GitHub.
Seems FastWinDirStat license don't match with using a closed source library, or something...
As for its actual functioning, it does as it says. Works much faster than WinDirStat