I've driven both on extremely hilly trails. A 4-wheeler is more stable. You would have to stand up and lean your thighs on the handle bars of a three wheeler to climb some of the hills that I climb. It's proper technique anyways. I also find myself doing that on my 4-wheeler, but I have a smaller 1985 two wheel drive Honda TRX 250 Fourtrax. Even machines this small have unbelievable power and torque in the rear, & it doesn't take much for the front ends to come up.
These machines are also designed for soft soil not pavement. A three wheeler traveling at a speed on pavement would not be very safe. Especially with new tires like Pryme has. They have so much grip, he can hang his machine upside down from the ceiling in this garage. I must need new tires for my machines!!!!!