I have a UCD 60 with a Saito 125 and a APC 17 X 4W prop. It performs very well and has all the power one needs for vertical work.

I had another with both a Saito 100 and a 91FX in it at one time. It performed well with these engines also but I like the new one with the 125 best. I only use full throttle when pulling out going straight up.

For 3D type flying, the four strokes have more low end torque and grunt which is important.