Hi David, are you referring to the older 120 models or the newer 122 size?
Not that it makes much difference, as the older/newer shapes, being narrower than the 133, will all feel more lively than a bigger, wider board. But for your conditions and skill level, I would really recommend mastering the straps, gybes before progressing to a smaller board. Especially as you say 20knots max and 6.5 to 8.5, rather keep that 133 volume for now.
Later, when your skill set has developed, can really recommend moving down one size, which should also fit those sail sizes and conditions well, once you have more active pumping to plane skills? good luck!