There is a lot to be said for both types of scenes.
Allow me to break it down for ya!
Gangbangs - On their most visceral level multiple cock scenes are about the fetish of a slut being in total submission. In a well done gangbang all of the sluts holes are filled and she is constantly being used up. She doesn't get a break. For the entire scene she has cocks coming at her and her job is to use her holes to pleasure them. The enjoyment of the scene is often found in how well the slut makes herself available to the cocks and is ready to transition from position to position and submit.
The cons to gangbangs/multiple cock scenes are often the visuals have a lot of man flesh. The enjoyment of the scene can be minimized if the viewer is focused on the visual of a hot girl having sex.
Multiple Girls - These scenes are obviously more about the visual. The camera can move between one slut to the next. The viewer isn't sure what kind of sex act or girl he will see next, especially in an orgy. One of the hottest parts of an orgy is when you see multiple sluts on their knees each sucking a single cock etc. The visual is full of different girls and different cocksucking styles.
The cons of course to a multiple girl scene is if the viewer is only really interested in 1 of the 3 sluts in the scene for example. That can lead the viewer to be a bit bored and frustrated if he is "stuck" watching a girl or act he isn't into.
I am a fan of both types of scenes but that is because my taste in porn runs the gamut from 1 on 1 highly stylized stuff like you would see at blacked.com all the way to the 4 on 1 DAP stuff you see here.