I am in the middle of a selection myself. I've had an Extang Trifecta for the last five years and it has been flawless. It's been on and off the truck over 20 times and it still does not leak a drop. I do spray vinyl protectant on it every other month. It has been through the summer heat and horrible winters without a hitch.
I was turned off by the Bak Flip solely because it rests on the back of the truck cab. I am sure there will be damage to the paint surface after time. Then I found the Undercover Flex. This seems like an improved version of the Bak Flip.
I was all set to purchase the Flex until I visited a local truck accessory shop in my area. Extang is having a special running up to November 24th. If you buy the E-Max, which is a vinyl version of the Flex, you get a BedRug mat for free. This was a no brainer for me. Saved $400 if I purchased the Flex and bedrug separately.
I am not worried about security so the vinyl is fine for me. If you are sure you want the hard cover, I think the Flex is a better choice.
Hope that helps.