I've had both, a Tonneau cover on a S-10, and a cap on a Silverado. The Tonneau cover was expensive and was a pain in the neck if I wanted to haul my dj equipment around. The cap was better for that. I didn't like the cap because it blocked my view while backing up, and it added extra weight to my truck. So I vote neither.
So when I got my Ranger, what I ended up doing was putting a toolbox in. It already had a bed liner in it when I bought it, and I'm happy with that. The tool box I bought doesn't go all the way down to the bed, so there's still plenty of room to carry long boards if I need to. The smaller sized tool box also leaves lots of room in the bed for carrying stuff.
So you have to determine what your needs are going to be and make your plans around that.
Here's a picture of my current set up......