I have taken deer with a scope and red dot (cheap simmons) on my shotgun. Scope (1.5 to 4.5 variable Weaver) works great on my slug gun. The red dot is good at closer ranges and you can aquire your target easily and quickly even if its moving, but once the ranges get farther (say beyond 50 yds.) I felt like the dot covered too much of the target area.
Scopes- more accurate, but may have trouble sighting on closer moving targets.
Red Dots- Great for closer moving targets, accuracy drops as range increases due to area of target covered.