Though pretty cool, if done right....the 50cc in 1/4 scale cubs and similar craft does require special care, especially if you plan to push the left stick fully forward
I think you are on the right track with the 30, 3d performance comes in at around 11-12lbs with the 30, depending how good of a 3d pilot you are. So your weight, and desired performance sounds about dead on for your cub and the 30.