Drop-in liners kinda suck. They are hard plastic that is super slippery, they actually remove a bit of space in the bed due to their size and shape, while trapping water and scratching the paint under them, increasing the chance of rust.
Spray-in liner is better in that it takes up less space and actually protects the metal under it and less likely to contribute to rust. Negative is that the Factory and Line-X type spray-in liners cost more and still kind of slippery. They also have a sand paper like surface that tends to damage delicate items, like say furniture.
If you don't need side protection, you should also consider heavy duty rubber mats that are designed for your truck. These mats fit & look great, protect the paint on the bed, offers a nice grippy surface so your stuff doesn't move around, you don't really need to tie anything down with the tail gate up, smooth/soft surface that doesn't harm anything, doesn't trap water, and usually only cost about $100. I had one of these on my F150 for 4 years, never cleaned it during that time, and when I traded the truck in and removed the mat, the bottom of the bed still looked new.
The perfect setup for an open bed is a spray-in liner with a heavy duty rubber mat. But if I was on a budget and wanted the best bang for the buck, I would get a Dee-Zee Rubber Bed Mat.