Oooooh... so tough. >.< I couldn't imagine my sims game without vampires, but casters are my preference. I adore both. Casters appear to be more supported than vampires, when it comes to actual game play, too, but that's also the nature of the life state. For me, vampirism seems more about the consumption of life energies and casters more about the manipulation of life energies... though you could argue that both consume and manipulate.
I think I will play casters more, but vampires and aliens that look like elves will always be close on my caster's coattails. At the end of the day, go occult or go home. ;) LOL