I have owned 2 stak 3 speed monster boxes and think they are built tough! Mine have never whined but the 1st box in the old truggy would sometimes come out of gear in 5.4. Shifting is tough while it is new but gets better as it breaks in. I have talked to the owner a few times and really like the service and product. Only thing I have not broke wheeling is my stak. Remember a time recently when I had the motor reved up and jaming it into rear wheel while being impatient.
The mini stak may work for your application and save some $. Atlas is great too. Good luck!