If you really want to know which one to get, consider this: The Pandora is much, much, much more powerful than the DS. It has a higher resolution than both DS screens combined, and it has analog controls and a small keyboard. It's also much more versatile due to the USB port, and has built-in TV out.
On the other hand, the DS has lots of games that you can buy in a store, that are written specifically for it, the vast majority of which will not run on the Pandora, whereas most Pandora games will be emulated games for older systems or ports of PC or GP2X games. It's much cheaper than the Pandora as well.
It's up to you to decide which one is better for what you want to do.