I don't know much about use of mugwort as a hallucinogen (my knowledge is mostly traditional chinese medicine based). It is used in hallucinogenic brews, but it would be pretty mild on its own I'd think. Any herb associated with psychic abilities is probably hallucinogenic. (It can smell an aweful lot like marijuana, which might add to it's reputation)
The leaves, or the wool from the leaves (if you have plenty of $) are used traditionally, I think picked before flowering from memery. I don't know anything about using the flowers (or if they are safe). Does someone else?
Often the flowers of a plant are seen as the ultimate "flowering" of the plant, (hence their use in flower essences) so they may be viewed as better for psychic pursuits (especially with the flower representing the head of the plant).
Mugwort and Motherwort are entirely different plants. Mugwort (artemisia vulgaris) belongs the the compositae family (as does chamomile, wormseed, arnica etc.), Motherwort (leonurus cardiaca) to the labiatae (along with mint, marjoram, rosemary, sage etc.).
From a homeopathic perspective the compositae family have an energetic association to injuries and shock, the labiatae to excitment and exhilaration. (Don't know if thats useful for you, but thought I'd throw it in)
Oh, and a link for motherwort
http://botanical.com/botanical/mgmh/m/mother55.html