That really is a moot point though....you're talking about different markets, especially with the DS.
The PS2 is only $130 now, has a big library of games (6 years worth), and had a couple huge titles this year. All of those things have an effect on why it's still doing so well. But you still can't compare it to the next-gen systems. The DS is also only $130, and is in an entirely different market (handhelds).