If you are teaching new beginners, the Mambo (or the Starboard Rio or Start M) will prove to be much better as beginners usually depend on the center board to keep the upwind.
If you are teaching more advanced students with all the basic skills already mastered (balancing on the board, uphauling, beachstarts, getting underway, tacking, basic jibes, planing a little, and staying upwind without a centerboard or daggerboard, then the GO 155-170 will prove to be alot more popular with your students.
The GO boards will plane earlier (due to the additional width) and have a significantly better overall range of use.
Hope this helps,
We already use the Start for beginners. But we get some people who've done some course already, and want something more advanced. But usually they cannot really do a beach start, so they haven't mastered the basic skills, but they still want something more advance.
I just don't want them to manage to leave the beach but then having to go and rescue them back