I found a study comparing humectants. Those who conducted it caveated that it's not perfect but I thought it's nevertheless worth sharing.
The sampling size is 5 individuals and they didn't manage to control humidity between three different classes of humectants. What was proven, is that nothing can beat glycerin. What seriously surprised me are the results for sodium lactate, urea and sorbitol.
Having said that, I would pay money for such information if the study was conducted properly.