Wii Fit or Wii Fit PLUS - definitely PLUS. It has a whole load of extra elements that raises the Wii Fit from good with some annoying bits to GREAT!
The following are in the Wii Fit Plus version only:
TRAINING PLUS: Have fun - 15 games; Perfect 10, Cycling, Rhythm Kung Fu, Driving Range, Segway Circuit, Birds Eye Bulls-Eye (Warning: you will look silly flapping your wings like a chicken), Snowball Fight, Obstacle Course, Tilt City, Rhythm Parade, Big Top Juggling, Skateboard Arena, Table Tilt Plus, Balance Bubble Plus, & Jogging Plus.
WII FIT PLUS: Build up a sweat - Choose a set routine of Yoga and Muscle work outs or build your own regime. Punish yourself with a long routine or just warm up with a short one - you decide. There are four different set routines Lifestyle, Health, Youth, & Form.
MULTIPLAYER: Laugh out loud - Compete against your family & friends. This is one element where the Wii Fit Plus comes in its element. And possibly one time when the family will be all together and enjoying themselves.
The Original Wii Fit stuff is all there: Yoga exercises, Muscle exercises, Aerobic exercises, & Balance games (skiing, snowboarding, ski-jumping, table tilt, tight rope, heading...)
FUN FITNESS FAMILY - the new essential FFF for you!