Re: Flats vs prefolds
You're right, a lot of it is how easy they are to wash and dry.
The other is how easily you can use them in many, many, many ways. The end result is usually more trim than a prefold in certain spots too.
In my sig is my favorite fold for stuffing pockets or laying in covers. Even with 2 flats inside a Flip it's more trim than any other solution I've found with equal absorbency.