Good choice on the Hammerite. I like it, because it doesnt need a lot (basically NONE), prep or sanding between coats. Its also forgiving when just painting over small scratches and scrapes. Also, if you get a little too close and apply too much, it the runs and orange peel mostly work themselves out to match the textured finish. AND it looks good...I think better than just basic black or silver or graphite...Which are the three colors I use, and I think any of the three would look good on a white truck.
As to bedliner, I would think my biggest issue would be that its got a gritty surface texture, and dust and dirt embeds itself, and would need to be scrubbed with a brush if you want it to look decent... Yeah, I know, a real off-road truck doesnt need to be clean, but then again...