Hey euphoria, ever notice how no two people look completely identical? How even among twins there are slight (oftentimes negligible) discrepancies in physical appearance and personality (among other things)?
Weed strains are exactly like individual people. All of the strains of the three species of marijuana vary in potency and offer slightly different highs. Certain qualities of a high tend to be attributable to a specific species (i.e. indica's known for body-heavy, "stony" highs, whereas sativa is known for the more psychoactive, heady, creativity-sparking highs, and ruderalis I don't know much about at all but I understand it to simply have a much weaker potency than sativa's and indica's are known for), but that's just a rule of thumb. It goes out the window when you start talking about hybrid sativa/indica plants.
To complicate matters even further, one strain of marijuana can have varying qualities to the high from one plant to the next. EVEN WORSE, a friend of yours could be effected by marijuana completely differently from you after the same session, completely freaking out while you're just chillin' there like "wtf bro, here, hit this."
This shit gets complex, man. In the end, though, street weed can be just as good as medicinal weed obtained from a dispensary. The difference is that street weed can also be complete shit, whereas dispensary weed is going to be quality-controlled to some extent. If they have low-potency product in a dispensary, it's for a damn good medicinal reason and almost assuredly of a high quality itself. Street weed? Fuck if you know what you're going to get unless you've got a reliable dealer that doesn't really sell to you off of the street anyways.