I love the drop-in
exactly because of the sliding feature. The spray-in won't let you slide heavy objects around easily like the drop-in, and isn't that what a truck bed is all about? I hate making things harder for myself when something is mega-heavy. Same reason I like the tailgate step, makes life easier on you and your back.
I never thought of throwing a rubber mat on top when/if needed for traction or less knee wear&tear, nice idea. To me that would be the ideal. Drop-in for ease of movement, and a rubber mat if/when necessary. They are also both cheaper, although I always get the best for my truck no matter the cost.
It's nice when I actually like the usefulness of a drop-in much better- and it also costs a lot less! That doesn't happen too often! That's what I call a two-for!