Obviously, you have texture and aggressiveness differences but I'm sure tha's not what you are asking.
I believe you are asking about the differences between, say, the VC (variable contact) and the DA (dual action) or "flat" pads.
I would say that in the end it's going to be pure preference that will make your decision.
The VC pads, are just that. They have a concave "pocket", if you will, in the middle of the pad that holds the polish as to prevent it from splattering everywhere. They also have a beveled edge to really get into some tight spaces. The only downfall of this pad is you aren't using all that much pad on the surface since it's not completely flat. This one, by the way, is the one I like the best.
Flat pads (DA). These come in different sizes, but the 6.5" is the most popular. This one's pretty straight forward, but if you aren't proficient with it, you can certainly fling polish all over the place and it can sometimes be a little hard to get into tight spaces since you can't really use the edges of this pad without tearing it up. I believe the new Sonus pads are based on this design but have a much better backing design so you can actually use the corners a bit and still have a pad afterwards.
Lastly are the Curved Edge pads. I believe these are for use with a rotary since there aren't any flat edges on this pad to use for a d.a. polisher.