I"m a big fan of powdercoating, but wouldn't powdercoat something that would see as much abuse as a truck bed.
Just paint it flat or matte black, then whenever it gets scratched or dinged up you can just spray and walk away. No way to really "fix" a scratched powdercoat or anodizing job except blasting and starting fresh, though I agree a black flatbed would look sweet!
If the aluminum is treated first, the paint can hold up really well. Experienced with and without treatment years ago when painting an aluminum canoe. Without, paint flaked off shortly after applying.
I would vote for paint, though I haven't considered painting mine.