Hi Chuck,
The 300 is the much better Trainer than the R22 in my view. No idea about the Gumbai. A few areas which make it this are these:
The non-governed versions mean that the student learns strongly the relationship between pitch and RRPM. The Robbie masks this via the governor. Even when the governor is switched off, the mechanical correlation in the power range involved doesn't really cause much of a drop or rise (which in the 300 must be manually compensated). This is a valuable lesson and training directly on a turbine misses it completely.
It has stick trim: this is great for teaching balanced flight and learning a realxed hold on the stick. You can get the aircraft balanced out and fly hands off (guarding naturally). It's also a nice step for later work with aircraft that have more sophisticated trim systems and AFC.
You can see the tip plane path. This is great for all sorts of reasons. It is of course a great aid to leaning to fly an attitude. But it is also good for demonstrating the relationship between pedal and disk displacement, and especially the effects of collective movement and disk displacement.
It needs more work to fly in a balanced state (and in general to fly nicely). The Robbie achieves balanced flight very easily and at quite low airspeeds. In my view this actually makes the 300 "harder" to fly than a Robbie. Sure the Robbie is flighty and the stick must be held and minded actively, but the R22 is a doddle to fly once you've got over its flighty nature and this masks lots of valuable hands on flying lessons.
I have also heard the argument that the R22 is the best trainer because its tolerance margins are low. To me, that's lobotomy logic.
TT