i used to work at a aftermarket truck accessory shop and we installed a few different covers. 99% of the ones we installed were made by a company called Truxedo; mainly the lo pro model. they are a soft cover that comes with a lifetime warranty. they install fast, look nice and stand up over time. i have one on my truck and i love it.
i also installed jack rabbits, pacer and a few other brands. the jack rabbit was nice because it was made of aluminium and rolled up into a box but there was a few downfalls. first if it got a little out of alignment it would pop out of the rails and get jammed up. second it had a big box that sat at the front of the box that housed the cover when it was rolled up. that sucker took up a lot of room. third it was a bear cat to get the rails to align and install in to the box.
the pacer models were a hard cover that could fold up into like 3 or 4 sections. they installed fairly easily but i didnt like the way they worked/looked. you had to fold it up from the back to the front to access the front of the box and when it was fully folded up you still had to reach under the rolled up section to access the front of the bed. they came out with a later model that allowed you to fold back the front section so you could access the front. i didnt like the look because it was plastic and just didnt look right on the trucks.
if you have any specific questions feel free to ask away. hope this helps