It all depends on the trailer/truck combo.
I have a qcsb, and a 30ft 5th Ragen toybox, spent the extra dollars for the sliding 16k Reese, and should have not bought the slider.
Not only do I not use it, I cannot use it. If I slide it back, it puts the pin box in such a position that if I would turn it would tear the hell out of my bed rails. The dealer told me I needed it to keep the bumper of the truck from contacting the front of the trailer when turning, not true. The front overhang of the trailer actaully comes closer to my rear truck window than the bumper does to the trailer, but both have ample clearance.
You need to do a "mock" hookup first, then you can decide what you really need.