both approaches to animation apply to 2D as well as 3D animation , what you use both of these for is more of less what your trying to accomplish in your project may it be in style or just the overall feel , both methods have there uses for example Pose to Pose gives a more solid structure for animating scenes and avoids the morphing of what ever your are trying to animate mid animation , while the Straight ahead gives little structure other then the relative outline of motion you have in your head while making the animation. This works especially for things like animated fire or the flow of water as both has very little needed structure , while if you where planning an animation of a character running to point A to point B for example this would probably be done in a pose to pose method .