Choose munna. While both may eat nightmares, nightmares are actually a staple of munna’s diet. To a drowzee, it’s more or less like eating a lot of bread. Eventually, you get sick on bread, and in any case, you really can’t survive on bread alone.
(I … honestly don’t know what to make of your analogies anymore. —LH)
If you meant to ask if you should invest in either at all, this may be something to discuss with your doctor. It may be helpful, certainly; it’s just that it’s always important to inform them of any steps you take to manage your mental health so they can factor it in to your course of treatment. That and if you experience extreme, daily nightmares, you may need other service pokémon in addition to a munna to help you manage your sleep.
But either way, yes, if given the choice, you should really choose a munna and not a drowzee.
The Pokédex According to Bill 