Don't be a sheep and let sales affect your decision, that benefits no one except Nintendo and Sony. If you have the money I suggest the PSP. Getting a DS is like buying a Toyota over a Ferrari just because it sells more.
In the end it's all about the games, the extras are the icing on the cake, which the PSP has plenty of.
Here's a recent comparison of the games on the PSP and DS -
PSP won in the following categories -
DS won in the following categories -
Overall PSP has the better games.