that championship mavs team had this one guy named shawn marion, dude only had to guard kobe, kd then lebron in three consecutive rounds. that and the fact that the team had tyson chandler, a future DPOY. all dirk had to do, and all dirk has ever had to do is play average defense for his team to be successful.
kobe on the other hand has changed playoff series because of his defense. literally every year there has been a pg who has dominated derick fisher, caused the series to go longer than expected then phil puts kobe on said pg, he slows him down enough for the lakers to win the series. from rondo, to chris paul, to ty lawson, to tony parker to aaron brooks (08 rockets ). 04 might not be the best example but we had the best defensive PG of all time in gary payton on the team but kobe ends up guarding the best wing players game after game including billups in the finals.
biggest weakness of kobe's defense is that he has no respect for role players, if you average 8 ppg and kobe is guarding you then you will probably find yourself open a lot because he doenst think you are a scoring threat. kobe's best defense has always been on dominant ball handlers and in his prime he is one of the best to ever do it on that side of the floor (dude has more all defensive first teams than duncan).
so yeah, taking dirk over kobe when starting a team is debatable. personally i wouldnt but i can see why someone would take dirk, but to say dirk is as good of a defender as kobe is laughable. i could print out pictures of every pf/c in the nba, tape them all to my wall and tell a blind dude to throw a dart at my wall and chances are the guy hit by the dart is as good of if not better defender than dirk