As Metal Kevin485 said, it depends on the kind of Alakazam you're using, and I'll tell you this: If you're Alakazam is build as a sweeper, you should use Psyshock since it will surprise Special Walls like Blissey, while Psychic will be a solid STAB for a Support Alakazam (which isn't that useful) to have.
Concluding, unless you're using a Support Alakazam, use Psyshock.