...which is better? I actually think Junction magic is better.
MP magic, as we all know (since it's so popular) allows you to cast any spell you have trained, provided you have the mana to do it. So with a sufficiently large mana pool you can cast quite a few powerful spells. However if you run out of MP you waste a turn to refuel, and often a potion may not get you back to 100% to continue casting. In real-time video games this is less of a problem (you pot and keep casting) but in turn based RPGs like FF it cuts into your DPS.
Junction magic, on the other hand, allows you to stock a ridiculous amount of charges of magic spells (100 max per spell), and so, theoretically, you can cast substantially more than a MP caster. Also, and especially at earlier levels, this results in quite a disproportionate advantage. I typically spend the first few levels purposely getting into encounters, drawing 100x of all available spells for each member, and then running away. I pot if I have to, then do it again. Once I have gathered a good amount of spells, I can go out and fight much powerful mobs for more xp.
Then there's the option of junctioning magic to your weapon, and have things like a firey sword or what-have-you. However this can easily be translated to things like materia from FF7, so I am not decided whether this is definitively better. It did eventually make me almost indestructable though, and to me that's a good thing.
What do you guys think? I haven't really spoke up much about MP magic, but it's usually limited to just casting, and doesn't have as much of auxiliary uses.