Well i have both systems, and i enjoy them pretty equally at the moment.
The ds seems to have more pick up and play games, where as the psp has more immersive games, plus I have been flying a lot lately, so those umd movies are pretty sweet on 3 hour plane rides
(and i've bought quite a few for 5 bucks at gamestop)
So i chose both...BUT, the new DSi. I haven't had a chance to get one yet, but i like what i see, that might tip the scales in ds' favor.