I don't think anyone can say for sure except Sony. The price drop didn't help profitability. In addition, Sony spends a lot of money on marketing. When is the last time you saw a DS commercial? I see PSP commercials all the time but not DS commercials.
The PSP has higher hardware costs, higher marketing costs and a poorer software attach ratio.
In terms of profitability, the there is no comparison. The DS wins in every category.