We pit the two most powerful occult Sims against each other! I love this question and even though I picked spellcaster by a small margin, I actually love them both.
I picked Spellcasters because I love how their powers relieve the drudgery of a Sims' existence. I love being able to concentrate on career goals without worrying about all the needs and repairs that spring up (I like contrasting spellcasters with my Sims who have to juggle all of this). I also love how both Glimmerbrook and the magic realm look and how the magic realm is it's own bubble of weather and culture and a safe space for spellcasters to gather and magic.
While the progression can be spammed to quickly ramp up there are other options. I can attenuate training and study and take my spellcaster through a process of mentorship and study that feels more organic and fun.
Slight disappointments are the gorgeous but empty greenhouse sitting in the corner of the magic realm, it would be an awesome place for magical plants and a Madame Sprout like herbalist character. I also wish there were livable lots in the magic area and even stores...maybe all clustered in the retail area.
I do also really love the vampires and the thoughtful balancing that goes through their skill tree. I like the sense of hierarchy in their structure and the way that Vlad contacts a new vampire initiate. Forgotten Hollow is gorgeously creepy and walking through that park or fishing in the middle of the night give me the exact chills that I want. I think the storytelling around Vlad's character in the builds is also really good, giving a sense of a vast unknown history. Also the brooding vampire social interactions are lots of fun.
Drawbacks (and generally true in the Sims 4) I want more lots to play with in the Hollow, more vampire specific commerce. I want Sims to be creeped out more when a vampire is seen feeding and hissing or is pursuing them through the fog of Forgotten Hollow (also startled more by spellcaster magic).
These are two great packs, not flawless, but both have enhanced my game in lots of cool ways.