Alright firstly by Price tag........ They cost "nearly" the same !
Wii - 199 $
DSi - 169 $
DS LiTe - 129 $

Secondly features -
Wii-

Got a motion sensor
Joypad is like a TV remote!
Cool 3D graphix
Wii Internet
Virtual Console lets you play NES, SNES, N64, Arcade, Commodore 64, NEOGEO, Sega Genesis, Sega Master System and TurboGrafx 1 games for Wii Points.
More here .....

DSi-

Got a camera and can store upto 3k pix!
Got a mic
Got a touch screen ^_^
Got 3D graphix ^_^
Portable
Got Photo editing and sound mixers
More here .....

DS LiTe-

Got a mic
Got a touch screen ^_^
Got 3d Graphix *yawns*
Portable *yawns again*
Can play both GBA and DS games
Longer battey life upto 19 hours! YAY!!
Stylus longer and thicker
More here .....

Thirdly........ U tell