For example, what would give more exp, a spell that costs 25 magic used 4 times or a spell that costs 100 magic used twice?

as far as I'm aware, it's the frequency of use that determines skill, which is why it's advisable to create low-level training spells and use them often. For example, to train illusion quickly, create and cast a 10 second light spell, and cast it wherever you go to boost your illusion skill.

this is one of those things that bug people in Oblivion. it doesnt matter what spell you cast, they all give the same amount of exp, at least within a skill. one destruction is the same as another.