The ability score loss is anywhere from 1 point to several DICE worth of points (e.g. 1d6, 2d4, 2d6, etc.) - it varies by the power of the spell, and what it's obvious effects are.
The chaosium, while more geared toward spellcasting, is also deadly, because most average PC's will have anywhere from 5 to 20 magic points, and can become stressed to the point of unconsciousness by a tough spell. Both systems would see about the same amount of spellcasting, if truth be told - though evil insane cultists would likely be tossing far more spells in the BRP version, because of items that can store magic points, and because they laugh at sanity loss (maniacally, with unerving gleams in their eyes).