Responding to your ancient thread! In the interest of making flying models, I took International Paper's data, noting that stiffness goes with thickness cubed, and made a "stiffness lightness score" by
dividing the relative stiffness by the density (area basis). The higher the score, the more stiffness per unit weight you get. (Click then zoom in to see the figures.)
That said, the relative stiffness figures in this table rest on an assumption of identical paper stiffness per unit volume throughout. Your finding that stiffness varies a lot by brand (and by paper type?) means there's another variable that's not accounted for in this particular table. Also if the longevity of tag cardstock is bad, that strikes out a lot of these options.
It may be that weight (density) is the primary concern for paper airplanes, but after that, looking at papers of comparable weights with different stiffnesses could be helpful for choosing.