If you get 150cm, you will get less fall off when doing full length shots. You can also keep the relative size a bit bigger, which means you can afford to pull it further back for a more even light distribution from 1 side of the group to the other. But the difference isn't all that big, so unless you KNOW you need the extra 30cm, I'm sure you'll be more than satisfied with 120cm. It's still pretty darn big and soft for most needs.