Ask a Thief Thread.

After you've maxed BoT, you can do tons of things. You're going to be training at Typhons, me thinks. And you can train on Typhons all the way till 4th job. So it really just depends on what you want to do. I personally went with Assaulter first, so I could one hit weak monsters whenever I'm travelling, but you can do whatever you want, basically.

As for your build, I suggest you get BoT to 18 first, since it then will actually outdamage SB. At that point, I suggest getting a bit of MG. After you have a lvl of MG you're comfortable with, you max out BoT. It's your main training skill all the way till 4th job, so the earlier you max it, the more you get out of it. The 50% extra damage is really worth it.

And I don't know if Magatia is good Exp, since I'm too high level to train there with decent exp gains?

:O good point

And even though assaulter does 30% less damage than SB, does it still do more overall since it ignores weapon defense?