I find that the visual difference between the two is negligable. The picture below is the picture I use for my environment map. It looks vaguely like the sun in the sky, and produces a nice metallic chrome effect. One reason marching pyramids is not used as often as marching cubes is that it can generate twice as many triangles as marching cubes does. Because the hardware texture-mapped triangle rendering is so fast on the SGIs, marching pyramids doesn't suffer performancewise as much.
Environment mapping helps to hide the course granularity.
C const.h funcs.h march. C movie.h particle. C particle.h random. C random.h texture.