I have a factory installed spray in liner in my Ram.....If you really use the bed of a truck, a drop in liner will be the better choice. I am far easier on a truck bed than I used to be, but even what I would call light to medium use will allow dents, and chip off chunks of liner leaving bare spots in the bed.
I have a full sized heavy rubber mat in my Tundra, it's great, no rattles and easy cleanup. Other than that, I'd go with the drop in liner and Rick's idea is economical and a good used liner is as good as a used hammer.