I have both flats and prefolds from GMD. I got the flats more recently, for the flats & handwashing challenge at dirtydiaperlaundry.com.
They're both excellent. I like the adjustability of flats and the different folds you can do. And they work great for stuffing pockets. And if you are traveling they are pretty forgiving of different laundry situations you might find yourself in.
The prefolds have served us well, though. I don't use them (or the flats) as much right now because DS is very squirmy, but I do still reach for them when we are at home.
Re: Prefolds versus flats