The Nintedo DS (Lite) is undoubtedly the most unique handheld for its generation. The creative new touch-screen control adds a new experience for games and works great when used right.
Although it adds a few extra features (Wireless Connections (and Wireless Internet Connection), Alarm Clock, Download Play, Calendar etc.), it gets to the point when you're dealing with games. Personally I dislike having a game system which tries to make itself an all-in one device with redundant features (e.g Cameras).
The difference between the NDS Lite and the NDS is mainly the backlight and the feel of the buttons. The backlight is a lot brighter, making it easier to play outdoors on dark nights or when the sun is shining too brightly. The feel of the buttons are also different, and personally I like them better than the NDS' ones because it's more of a "push" than a "tap" when you press it.
The variety of games are great, from Educational (Mainly to build up certain aspects of your brain, to mention one, Brain Age) to Violent (Not so violent though). The Nintendo DS caters to more age groups (despite criticism of only having childish games) than other handheld systems with their wide variety of games.
In conclusion, the Nintendo DS is the choice for you if you want a new gaming experience, and a fun time wherever you go.