DS has a lot o good games (like mario kart, the legend of zelda and the world ends with you) and the touch screen... but too many of them abuse this touch thing and gets annoying... PSP is good too just because it has Monster Hunter and god of war
I like both of them but DS has more games that appeal to me but the psp has only two games that I would like which would be Patapon and Daxter so DS has my favor but not the DSi.