I don't know about scholar either, because I stopped playing by the time the expansion with that in came out. Blue mages are hard to master since you have to wait for the enemy to use the move on you too. As for the other two mages, it's very expensive. Not only do you need the right level for the spells, you have to buy the actual spell scrolls for a crap load. Plus, if you don't have all of the spells you can up to your level, you won't get asked to party as much because you'll be gimped, so you have to farm a lot before leveling. That was in XI, anyway.
Corsairs are pretty neat, since they can do AoE buffs and stuff. They also had some dice rolling mechanic which I've forgotten about, but it was neat.
By the way, you'll be able to have a sub-job. At lvl20 you can get the sub-job in XI, which means you can have skills from both areas. However, your secondary job is capped to half of your max level in XI.
eg:
White Mage is lvl 40.
Black Mage is lvl 30.
If you have BLM as your main job and WHM as your secondary, you'd be BLM30/WHM15. Also, you can have one character for all jobs. You simply go to the mog house and change which job you want to level, so no need for multiple characters. I'm saying all of this because it's the same team who worked on XI who are doing XIV, so I have a feeling a good deal of this will carry over.