Meh. Define what is a "better" career?
They are different players with different value. Dirk is mostly a PF, occasionally a C or SF. Pierce has been both SF & SG a lot, occasionally a PF.
I think these things come down to how much you value certain things. They both played almost the same total minutes so far in their careers (right around 40K).
In raw stats, DIrk has wracked up just slightly more points and a lot more rebounds (9096 to 'just' 6651 for Pierce).
Pierce, on the other hand, has wracked up a lot more assists and thus has been responsible for more total point generation (about 33,621 compared to about 31,569 for Dirk).
Notably, Pierce' scoring rates were nearly identical to Dirk's through much of his career right up until Ray & KG arrived, when it dropped from around ~25ppg down to ~20 ppt, where it has stayed.
DIrk also finally won a title when he had more help and his own personal scoring dropped down a couple of points per game. Dirk's rebounding that season also was down to 6.7, from a career avg of 8.2. These are just indicators that it takes a solid team to win a title.