I think the rendering with the lighter background is better because the reflected light from the background helps define the shape of the spring, and color-wise ties the reflected color from the top of the metal together with some visible element from the scene, making it less artificial. Also, the lighter background produces a clearer silhouette for the whole part, making it more recognizeable. This would be good if it were a catalog item that had to be shown ias small thumbnails too.
//well, on second inspection it seems about the same, the top blends in to the background in the right image, and the bottom blends in on tha left image. Ideally, perhaps the background would have a slight gradient that could yield a good contrast on both top and bottom of the part.
The faint reflections and shadows in the ground plane also help to define it as a physical object on a surface rather than a physical object floating around in outer space.