I've had drop ins, both under and over the rail, Line-X and I've had weather tech bed mats. My current 2019 Ram 1500 classic, SLT package came from the factory with a drop in. Penda-liner I think? I added a cheap rubber egg crate style mat from Home depot over the top of that to keep things from sliding all over.
Myself or any of my family or friends that had drops ins never had rust or rust through caused by the drop in. They for sure will scuff and dull the paint in spots, but I use my trucks so I don't care about how the paint looks in the bed.
I will say I loved the weather tech mat in my 2017 Ram 1500. I would have transfered it to my 2019, but it has the shorter bed.
If I had a choice of a liner or the rest, I'd go with a quality spray in and add a mat over that for impact resistance.