Depends what you want to do... The inverted pendulum will have it's own ARC control for balancing the robot on two wheels... It will remove all the complexity of having to code the movements such as balance, forward, reverse, left and right etc, yourself.... You might be able do the same with the 4 in 1, however, you'll have to write the scripts under say, the custom Movement Panel to do all of the balancing, forward, left, right etc yourself... One way will be easy, the other way is harder....

Again, what did you have in mind that you wanted your robot to do? If you want to balance on 2 wheels like a segway then buy the inverted pendulum... If you want your robot to walk without falling over or if it does fall over, know how to get up... then buy the 4 in 1....

The reason the inverted pendulum sensor was created was because the accelerometer and the gyro on the 4-in-1 could not provide information to the ez-b fast enough to balance a robot on two wheels (The robot would fall over before realizing it had to balance itself). So the inverted pendulum sensor was created with an onboard processor to increase the processing speed so the robot could balance. I could be incorrect in a few places, but I think that's the general gist of it.