I have tried both and the conclusion is clear. You do need a bed liner if you are going to "use" the truck for a truck in any way. In addition, this last truck I purchased, I had a "sprayed in" bedliner in there and that is by far (while more expensive) the best way to go.
I love it and will make sure in the future to budget for that item as one of my first options in my new trucks. With the sprayed in liners, things just don't seem to slide around and also it is better looking and water doesn't get around the liner and down in the bed to rust the underneath area. There is just no comparision on the aftermarket and spray in liners, the spray on liners are heads and shoulders above the plastic composite slide in liners. (always hated those bolts in the metal truck bed to clamp it down)