While the choice between a lawn tractor or a ZT type of mower is a personal one of choice, the performance and comfort of mowing become concerns - and to traverse slopes is important from a safety and performance standpoint. That said, we mow around our 20 acre property with lawn tractors, and I have learned how to handle slopes with them. All the tractors are 4WD - but that is 4WD either in or out, and what we have found to take the cake is the New Holland TZ 25DA series. Not sure if they are made anymore, but these are heavy duty tractors and ours has the Sensitrak option. This is where the operator doesn't have to switch between 4WD and 2WD, but like AWD on a SUV, the tractor senses traction and sends power to the front wheels as needed. This is my wife's mower, as she was never comfortable with mowing slopes on our property.
As far as falling off of a tractor on a slope - my JD tractors all have side rail hand grips - and they are handy when on a slope to hole onto for stability. This is a cheap addition which the user can add - and I'd recommend using the factory handles from the OEM as they fit well and work nicely!