Both are good from different aspects...
PSP is a better media machine, 2GB cards can be bought for about £35, which gives you enough to watch on planes, built in browser and RSS feed etc etc. On the downside the games are frequently poor, although there are a few good ones out there.
DS is a better games machine, much more fun to play, but less of the media capabilities.
Personally i have both and would recommend the DS if you just want a fantastically unique games machine, but if you want it for travelling etc then the PSP has more worth.