The stark difference between the purple, techno-looking background for the robot and the organic, rolling clouds behind Sonic really emphasizes the difference in the characters and reminds one of the folklore of John Henry.
The ominous shine of the robot's purple eyes and the blinding brightness of its rocket make it seem dangerous while Sonic's soft blue and clear eyes make him stand out from the rest of the image as a hero.
That Sonic is behind the robot but still looks confident draws up images of Henry again, picking up his second sledgehammer.
For these last words; excellent work.
The texture in the background matches Sonic well, as, they both have a painted feel in my opinion. The sparkling pink towards the bottom contracts with the the more matte oranges and reds of the rest of the image. However, this does not take away from the image.
In comparison, Metal Sonic looks more solid than Sonic. By this I mean, Sonic's proportions make him look a little chunky, kiddish, and small, though I must admit this does give him a level of cuteness.
The tracks he runs upon are well constructed and the colours there are very nice as well. The lines in the tracks are clean and very nice on the eyes.
The only think about this image that distracts me is the singular lightning bolt in the background. Because the cloudy and ominous looking sky takes up most of the background, the small, singular lightning bolt feels a little out of place for me. I feel like it should be extending closer to the bottom of the picture, or that there should be more lightning in the background if you would like it to be present (This could be only me though, I get distracted easily by little things like that).
Overall, very nice picture with nice colours and composition. The twist you gave to the appearance of the original level was a good one indeed, and I like the final product.
I was just about to comment, "All thats left is Metal Sonic" and then I spotted him and I was all WHAT NO WAY WHOAA...you know, some kind of cliche 90s reaction to seeing cool stuff haha
I love the colors used in this picture and the shading, it looks really cool. Also, I like the effect you did of Dr. Eggman melting the platform behind Sonic, though I think adding him in the picture itself would've added a little extra bit, it's still great all the same.