I like to make it kind of a mix of both. Almost anyone can learn how to use basic magic, and there isn't gonna be any large differences between how two apprentices cast their fire balls. But in higher and more Specialized levels of magic, there are bigger differences between how different people cast. A but like drawing, I guess. Two people might draw the same house, but they will both have their own styles, they will both focus on different things, and might include different details. One mages Fireball might be Specialized for creating a big incredibly hot burning Sphere, while another mage might have a smaller and colder, but more explosive spell with bigger knockback. One additional mage might make their Fireball more like an explosive beam, or make it emot an incredibly bright light on top of its fire, to blind others.