Here are some other differences between the F1 and the Flex:
1. When the panels are folded and stacked, the Flex comes with straps to hold the panels in place. The F1 just has D-rings, so you will need to buy and use bungee cords.
2. The F1 has two bolt hole covers on the panel by the cab, the Flex does not which makes it a bit nicer looking.
3. The Flex has two additional (vertical) rubber pieces in the seals that are between the panels.
4. The Flex is 3/4" thick, the F1 is 1/2". (Bak's website used to say that the F1 was 5/8" thick).
5. The Flex is rated at being able to hold 400 lbs of distributed weight, the F1 is rated at 500 lbs.