Since I'm a guy, I guess there's really only one answer possible for me. Still assuming I could be the world's first male Slayer I'd still go for witch (or wizard/warlock). Personally, I don't see the Slayer's limited life-span as a problem, that just depends on how you use the abilities that you're given. If you chose to ignore your destiny and avoid supernatural encounters, there's no telling how old you could become. The main reason I would go for the magic-user, is that there doesn't seem the be a cap on the maximum power-level. Slayers can only get so strong and they don't gain extra abilities beyond those they start with; sure, they continually increase their skills of strength, speed, agility, and focus, but even that goes pretty slowly. Magic-users are not limited by what their physical bodies are capable of. When they increase in power, they can add a whole arrey of new special abilities, like pyrokineses, telekineses, teleportation, flight. Moreover, as we saw at the end of Season 6, a sufficiently strong magic-user can augment his/her body in such a way that it is on par physically with a Slayer.