And yes, you can eat the stems, the caps are usually regarded as having a higher concentration though.
Caps and stems are approximately equipotent, in terms of psilocybin content% by mass. In fact it is often the spores and gills which are blamed for unpleasant taste or gastric distress, so stems are actually more desirable than caps. Spores do not contain psilocybin, and gill fragments are very low in psilocybin content, so "mushroom dust" at the bottom of the bag is *not* potent.
However, depending on the species and growing conditions, the absolute mass (grams) of mushroom flesh may be concentrated in the cap or stem.
For example, Panaeolus copelandia has a very frail stem that weighs just a few milligrams when dried, so there is obviously more drug present in the cap.
On the other hand, certain varieties of Psilocybe cubensis will grow a thick, meaty stem topped with a tiny, almost spore-less cap. So all the drug is in the stem.
I've seen both thick and thin stemmed Psilocybe cyanescens, as well as specimens of Psilocybe cyanofriscosa that had nearly a gram of dried cap mass.
So remember: mushrooms are not weed! Eat the whole thing and sprinkle the spore dust on your garden compost!