If you are going to fly on floats, leave the 150 on...it is not that much more power than a 120, (and the 120 does have have that much more than a 91) we are talking fractions of a horsepower here.
The 150 would allow you some flexibility of swinging a bigger prop at lower RPM..If you compete at higher elevations, or hotter climates, you won't be hurt so much by the power loss..... Remember the throttle works in both directions....!