This is a tired old debate that I wish would go the f*ck away.
Anyways, the Genesis may seem more impressive, but that's because Sega was able to secure more talented third parties with bigger budgets, and they could devote more time to messing with the hardware to create nifty tricks. The only advantage the Genesis really has in the graphics department is the second background layer. It's an important advantage though. Also, the Genesis can do tile flipping (this helps for memory usage, a major problem on the Genesis), and from what I understand, supports 8x8 sprites, whereas the TG16's smallest sprite size is 16x16 (may not seem important, but using a 16x16 sprite for bullets in a shooter seems wasteful). Things are pretty close between the two consoles, and zealots on both sides like to point out this or that advantage. I'll give a brief zealotry rundown:
Genesis Zealot: The Genesis has 80 sprites!
Turbo Zealot: Yeah but the Turbo can do 32x64 sprites, whereas the Genesis can only do 32x32 tops!
Genesis Zealot: The Genesis has two background layers! Hah, beat that, Turbonerd!
Turbo Zealot: So does the Supergrafx! And it has twice the RAM! Hah back at you!
Genesis Zealot: Hey that's not fair, no one has a Supergrafx!
Turbo Zealot: Hah! Sucks to be you!
Genesis Zealot: The Genesis has SHADOW MODE!!!!!!*&^!@$#^^
Turbo Zealot: The Turbo has more colors already built in without the need for some stupid hardware hack!
Genesis Zealot: The Genesis has 32 megabit games!!!!!!!!!!!!!!!!!!111111111
Turbo Zealot: You need all that extra memory for the wasteful CPU your console has! We don't NEED cartridges that big! Besides, our Street Fighter II kicks your Street Fighter II's stupid ass!
Genesis Zealot: Sega's games on the Turbo look like ass!
Turbo Zealot: WELL NO DUH! They had to make them look like crap on superior hardware (Turbo) so they could sell more of their inferior hardware (Genesis)!
Genesis Zealot: Well suck on this! The Genesis has a 16 bit CPU with a higher clock rate! HAH! Sucks to be your old 8 bit turtle!
Turbo Zealot: The Turbo's CPU is more efficient than your wasteful RISC wanabe CPU! It performs better than your Frankenstein of a CPU!
Rarely do the tile-flip and 8x8 sprite size advantages of the Genesis come into play because the common zealot knows nothing of these details...these were not pimped by magazines back in the day so zealots never touch on them.
Coming right down to it, neither machine is the clear-cut winner, both have their strengths and weaknesses. The Genesis shines in parallax, the Turbo shines in colors.
And don't bother with the speed argument...games run at 60FPS, plain and simple. The speed of the game is controlled by the software; if I make tiles scroll at 2 pixels rather than 1, my game is going to look twice as fast. This is the concept behind "speedy Genesis games" like Sonic...variable tile scrolling rates make the game faster...it has nothing to do with that old media buzzword, "blast processing".
Standard resolutions:
Genesis: 320x224 viewable
Turbo: 256x224 viewable
I don't know how high the Genesis can go but I do know that the Turbo can do 512 pixels across and about 263 or so tall.