No experience with the Bakflip, but I hear they're good. I have an Extang and love it. It's very similar to the Bakflip, but does have a few differences. Both covers are hard folding covers so both are pretty secure, and durable. The Bak covers, from what I've seen, are a smooth black finish, so the previously mentioned acorns or hail will make noticeable marks. The Extang tend to be a textured black finish so it's less likely to show those marks, though some will prefer the look of a smooth cover rather than a textured ones. Much like the choice between G2 and F1, Extang have several options (Solidfold, Encore, etc) so I may be making more variables for you. Something to consider though so you're not just looking at one brand. I think they're all priced about the same so it's not a decision you want to just leap into.
FWIW, mine is the Solidfold which has no rails or anything. Install is seriously a matter of securing 2 clams with wing nuts at the cab end, and 2 clamps with springs on the tailgate end. The entire cover can be installed or removed in less the 5 minutes by 1 person with no tools. In the end I don't think you can really go wrong with any of them. They're expensive but IMO very much worth it, compared to the crappy canvas covers I've had back in the day.
Here's the best picture I have of it on my computer. It's from when I tinted the 3rd brake light on my '07, but it gives you a good look at the finish on the cover.