Why Magus? They are severely underpowered compared to summoners. Mine (job level 99, normal level 46) can easily deal 2000-3000 damage to everyone, and that's not counting elemental weaknesses or using Leviath/Bahamut. If your looking for a good AoE damage spellcaster, Summoner wins in my opinion.
I'm thinking of changing my Dark Knight to Ninja, but I'm not sure. Also, Black Belts kick major ass. Mine deals far more damage than my knight, is faster, and is steadily gaining her way to 9999 HP.
It really is just personal opinion in the end though, since Maguses (for instance) can deal status ailments.