I think different systems work better for different ideas.
Let's take the example of some kind of mystical mage, who draws on an internal supply of willpower, or magical energy to call forth magical effects. Obviously, a MP system works best here.
But then say we have a book mage, who studies speels from a tome, and commits them to memory. After he recites/enacts the spell, the efoort could cause him to forget how to cast the spell, unless he took a considerable amount of extra time to memorize it (learning it in two 'slots' or whatever) obviously a memorization system works best here.
But then we might have a cleric or priest, who receives his powers froma divine patron. This patron may grant him a certain amount of spells a day, possibly letting the priest choose them, possibly deciding for him. Once he has cast all the spells that his diety has given him, he wouldn't have 'fergotten' how to cast the spells, but would jsut not be able to, a memorization system works best here as well.
You could then do neat things like the wonderful idea of blood magic, and make an action (sacrifice) give the player MP points when it is completed, and this would allow him to have the MP to spend on various spells.
I don't think any system is better then any other, or more realistic, it just depends on how you want the mechanics of spell casting to work, and how easy you want to make it for the player. MP doesn't always make it easier for the player, you could have an MP system that worked very slowly, and a Memorization system that gave you plently of spells per day.
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He stood, paralysed as the beauty of the pattern unfolded under pressure, the hectic chaos forming designs more intricate then the smallest smile.