What actually differs
A slab machine has small solid tires, two-wheel drive, a few inches of ground clearance and a narrow deck. It is built for a finished floor and it is the cheapest way to work at height.
A rough terrain machine has large lugged foam-filled tires, four-wheel drive, around a foot of ground clearance, often outriggers, and a much heavier frame. It costs roughly a third more to hire, and it will drive across a site that stops a slab machine dead.
The rule that avoids the mistake
If the concrete is not poured, book rough terrain. Compacted base course looks like a road and behaves like one until it rains, at which point a slab machine is stuck where it stands.
The economics support it too: the price difference between the two is small next to the cost of a wasted delivery, a lost day and a second machine.
Where slab machines are the right answer
Interior fit-out, warehouse maintenance, retail, hospitals, schools, anywhere with a finished floor and a door to get through. In those settings the rough terrain machine is the wrong choice: too wide, too heavy for some floors, and it will not fit.