It is extremely difficult to say which one is the best and which one will go farther as they are both still so young.
I just read again an article from july 2000 regarding Shuei Peng and Su Wei Hsieh in the French "Tennis Magazine".
At that time, based on results of both players in a European tour with the ITF, everyone thought that Shuei had a far better future than Su Wei (though they were also certain that Su Wei would do well).
But for the time being, Su Wei is ahead of Shuei.
From the way I have seen both Angelique and Su Wei play, it would seem that Su Wei is more able to adapt her games to her opponents and take advantage of their weaknesses while Angelique is bashing the ball... in or out. This would explain according to me why Su Wei has had more consistent results.