The equation ...
Rose Levy Beranbaum says
for 1 tsp instant, use 1-1/4 tsp active dry
for 1 tsp active dry, use 3/4 tsp instant
She also points out that, because active dry yeast contains more dead yeast cells than instant, it is preferred for some kinds of dough, such as pizza dough, because the dead cells contain glutathione, which relaxes the gluten bonds and makes the dough easier to stretch.
The dead yeast cells also add a desirable flavor (but she doesn't define "desirable" -- a slight yeasty our sour flavor?).