For sliders, I like the bedliner option. For one thing, I am standing on the sliders like step rails way more often than I'm bashing them on rocks (which is, sadly, pretty much never). The grip that bedliner gives means I'm not in danger of slipping off and breaking my ankle in the often wet weather we have here. I got my sliders professionally Rhino-lined before installation, but that is an expensive option for most.
For other parts, Plasti-dip if you're not sure you'll like the color change. If you are gung ho about the color change, then go with spray paint, but prep, prep, and when you think you've prepped enough, prep one more time. That's the key to a good rattle can finish.