which is better in FF tactics advance, a warrior or a monk?

Of those two specific classes the monk is the better choice on the whole.

Warrior's aren't bad, and you certainly will want Monkey Grip, and maybe Shieldbearer, but their abilities are not all that special. It is generally better to kill a target than it is to weaken it. You will almost certainly put the Warrior class away once you have access to better classes. (Though again, look into learning Monkey Grip).

Monks are one of two classes that can permanently kill undead. They are also very counter happy thanks to the ability known as "Counter" of course. However, like the Warrior, the advanced classes of the Bangaa are simply better, and you probably won't stay in this class for long. (Though don't forget to pick-up Counter and Reflex, both are very useful.)