Yoshimo's thieving skills go up faster than Jan's, but that doesn't make him a better thief. Jan has equipment that puts himself way ahead of Yoshimo, plus he's a mage, plus he isn't a *beep*, plus he isn't that annoying, plus he has some very funny dialogue like "You'll go far you know. Even further maybe, than Uncle Bifflechips. Though he was flung by a catapult, so I suppose I shouldn't count that."
Anyway, go with Jan.
------------------
GameBanshee, where I shall be a moderator, one day...