If you have two cars with a slide carb and a rotary carb side by side, you'll immediately notice that the slide carb reactes quicker. You can even hear it. The rotary carb also needs to travel farther in order for it to be opened to the same degree that a slide carb is.
Also, a rotary carb forces air to snake its way around the carb while a slide carb allows air to flow down in a straighter path. If you want a picture verifying this statement, rccaraction.com has a great picture of this fact in one of their articles... I don't quite know where (it's been a while since I've visited there)
For on-road, get a slide carb for almost any condition. If you are a tuning freak and expect to race somewhere that is very slick, consider a rotary carb, but otherwise, leave that type of a carb for offroad duty.