I had also considered the Extang and Lund line of covers and even emailed Extang direct asking some questions on which of their covers would fit my Titan, since I had the under-the-rail factory bedliner and the plastic bed caps.
I was told that none of them would work "out of the box" unless I notched out some of my bedliner, so I decided NOT to purchase an Extang.
I instead went with the Access Limited Edition cover by Agi-Cover (
http://www.agricover.com/index.php/products) which mounts on the inside of the bed rails and doesn't interfere with the bedliner or the bed caps. It has the tension dials that you can adjust to tighten or loosen the cover, so no more strugling in the winter to pull the cover to close it and no snaps. It also comes with both front and rear element seals to keep water from entering the bed of the truck when the cover is secured. The bows are actually built into the cover, so when you roll up the cover, you have access to the entire bed (another selling point for me).
Cost was around $410.00 shipped (from
http://www.4wheelonline.com), but I really like the look of the cover and the installation was pretty straightforward. They have another model (Lorado) which is a low-profile cover that is almost flush with the top of the bed rails. The tarp and the mounting hardware all come with a lifetime warranty.