a rotary carb has a barrel that spins to open the carb and allow air in. with a slide carb, the barrel slides out rather than rotating. In theory, a slide carb is better for onroad racing, as it allows more throttle response at low rpm, and at max rpm, but not as much in the middle. a rotary carb is better for off road as it allows less power at the low end and prevents excessive wheels spin. this info relates to small blocks only and not big blocks or 8th scale buggies.
a slide carb for off road is good, however, for less powerful engines, as it wont over kill them with torque, yet allow more crisp throttle response.
one thing to be aware of, though, is that to use a slide carb engine, your radio must have EPA.