I've had an F1 cover on a few f-150's for the past 9 years and it held up rather well. Now that I have a F-250, I'm in the market for a new tonneau. For those that have bakflip covers, whats the difference between the F1 & MX4? Not much comparison info is out there, so I'm assuming its just a fiberglass vs aluminum material for the panels? Thoughts?