[crimsonson]

Yoshimo is the closest to pure thief an NPC can come in BG2. His battle skills is better than Jan's. Jan's Con/HP is horrible. Yoshimo's backtab is better. Jan's hide in shadow is worst than Yoshimo's. Jan cannot wear an armor if you want to use his mage spells.

HOWEVER, Jan's is a good secondary mage spell caster.
I think there is enough equipment and poition to increase any of thieving skills (rings, boots, capes, etc).

Yoshimo fits in the story line better.

IF you want Jan, I suggest to pick him up early since he is a thief/mage you will need a lot of XP to get him to higher leave where he is more efficient.

I do thinks Jan's native equipment is overrated by a mile. You can pick up a lot more bows that offer better damage and hit rate.