If you don't already own a DS go and buy a DSi you won't regret it. You'll love the games on DS you get all the best games Nintendo/Microsoft have to offer. If your a Microsoft or Nintendo fan then the DS is the console for you. Whether your playing Rare classics like DiddyKart:Racing or ZooTycoon or AgeOfEmpires the DS is definatly the console for Microsoft fans. Of course Nintendo fans get all their favorite franchises living out life on the handheld and as a Nintendo fan you won't be dissapointed.
If you already own a DS:Lite, then it is not worth the upgrade. I myself bought a DSi upgrading from the lite and to be honest I used the camera once and have one downloadable title. Whoopdefreakingdo. If you already have a DS (The best handheld on the market) then by all means buy a PSP. My PSP is pulled out every time I'm going a long distance, I use it as an MP3 player, watch UMD's and play the odd game. Of course for shorter trips I rely soully on my DS. But sometimes I feel like watching a movie or playing some games not found on DS (PowerStone/FullAuto:BattleLines are my fav's).
But as I said if you don't own a DS, go get a DSi you won't regret it, if you already own a DS:Lite don't get a DSi it won't be worth it. If you own the origional DS yes by all means get a DSi, its worth the upgrade from the origional.
"In God We Trust - In Games We Play " - Joel Reimer