Hey, scuzzo. Nice to see you are posting on these boards now!
The reason for it is because the TG16 had better hardware in terms of colors. The TG16 had a larger palette of colors and could display 256 colors on screen at once while the Genesis had a smaller color palette and could only display 64 colors on screen at once. If you do screenshot by screenshot comparisons for games for both consoles, you'll see that the TG16 always looks more colorful and vibrant. In fact, PC Engine FX even shows this with their
Lords of Thunder comparison, in which the original Lords of Thunder for the Turbo Duo is compared with the Sega CD version. There are probably other sites that compare other games as well, but can't think of any off of the top of my head. I was actually make a big .zip file that compared screenshots of many games once, but deleted it.
You should also hear the sound of the TG16. It can be debatable of what sounds better between the TG16 and the Genesis, but I generally much more prefer the TG16's sound because it's a lot more smooth and crisp compared to the Genesis' loud and trashy sound.