The two are very different. The Bosu is like half of a stability ball attached to a flat base. You can set the base flat on the floor and do stepping type moves on the ball part, or kneel on the ball and do weight work, or do some very effective abdominal work on it. You can also turn it over so the ball part is on the bottom, and hold onto the base to do core work (some valiant souls actually stand on the base with the ball down, but that's way to unstable for me!).
The Reebok core board is a platform on a base with short legs that are splayed on the ground. The platform can be set to three different instability levels, and it can move front-to-back, side-to-side, and swivel. You stand on it while doing squats, or throwing punches, or hold on to it while doing push-ups.
The Bosu now has about 6 differnt workouts that you can buy (cardio and strength), while the Reebok core board still only has 4 videos (one of them being a compilation of the other three).
The Bosu can be used to take a basic step workout (not the speed or torque of Cathe's) and take it up a few notches. Workouts like the Firm or Kathy Smith's Step workout work well on it.
The Reebok core board can be used as an unstable surface to stand on while doing squats, lunges, standing upper body work.
