DS Lite:
+ Cheapest of the lot + Backwards compatible with GBA + Huge support from 3rd parties (flashcarts included) + Battery life is good - Only WEP security encryption - No DSiWare and cannot run specific DSi Games - No Cameras or SD card slot
DSi: + Slightly bigger screen + Cameras and SD Card slot + WPA security encryption - Shortest battery life - More expensive.. - No backwards compatibility
DSi XL: + Massively bigger screen + Better battery life than DSi - Loss of portability in a sense due to screen size Otherwise read as DSi