The PC Engine version has flat backgrounds except for the floors and sometimes ceilings that line scroll as well as various objects that scroll along with the foreground (same as the 3DO Super Street Fighter II Turbo port). Otherwise the graphic quality is roughly on par with the Super Famicom version and the two look very similar, except that the PC Engine generally uses more vibrant colors overall.
The Mega Drive and Super Famicom versions have a similar amount of background layering, except that the Super Famicom has an extra layer on some stages. However, the Mega Drive version has noticibly less color and less shading overall, with some details missing altogether. The Mega Drive version makes up for this with some extra details and animation not found in the other two ports.
You can compare all three ports as well as the Arcade in the following video:
The full breakdown of each stage can be viewed on the the following pages:
Over the years, I've occasionally heard people say that they believe that the SNES/SFC version has the best line scrolling effect. This opinion is likely influenced by the perception put forward by magazines that the Super Nintendo does all "warping" type effects best because of Mode 7.
After carefully comparing high quality footage recorded from real hardware, I've come to the definite conclusion that all three ports have an identical line scroll effect. Both in how the image appears at each stage of scrolling and the consistancy of the animation. This should come as no surprise since not only does this effect require no hardware rendering (Mode 7 or otherwise), the pixel by pixel method would look noticeably different if it any version staggered it with thicker increments.
You can compare the line scrolling of all three ports as in the following video: