Most everyone above is correct...there is virtually no difference between potency of the caps and stems. Anyone that disagrees can feel free to save their "less potent" stems and I'll gladly take them.
As for the statement that Cheese made above reagarding the bluing areas correlating with psilocybin...this is not an urban myth. The bluing in shrooms is caused by the decomposition of psilocybin, so whereas is doesn't actually show where the active psilocybin is, it does show where the decomposing psylocybin is...however, this tends to be the same thing because the areas with higher psylocybin decomposition are also the areas with higher psylocybin content.