I know you said fold-up... but I've had both. The roll-up is way better in my experience.
The roll-up velcro's to a rail along each side, which keeps dirt out better. It closes with a lot of tension to keep the top tight and better looking. Also you can open it exactly as much as you need. My fold-up only opened 2/3 of the way because the last third was clamped to the bed rail.
Hope it helps!