The Genesis had 6-bit sound and the Super NES 8-bit.

The Genesis had two sound chips, one used FM synthesis and the other
used subtractive synthesis. One of the chips also had the capability
of playing digital audio, but it wasn't really useful for music, AFAIK.

FM synthesis = digital synthesizer (does bells and metal really well)

Subtractive synthesis = analog synthesizer (more organic and electric)

The SNES used wavetable synthesis. This means taking short sound files
of whatever you want, and playing them at different pitches.

Comparing the sound chips is a bit apples-and-oranges. On one hand,
since the snes was just a bunch of samples, you could have any
instrument you can record in your song. On the other hand, the sounds
had to be so small and short, that they end up sounding pretty damn
synthy or weird no matter what you do, a lot of the time.

The Genesis music will always sound more fake and video gamey,
but it will also be smoother, and in a weird way more organic (despite
sounding more fake) because the instruments are being generated
dynamically by the sound chip.

An example of this, let's say you had a piano sound in the SNES. If you
play it at higher pitches, the sound becomes shorter and if you play it
at lower pitches the sound becomes longer. This sounds weird
computer-like. On the Geneis, it will smoothly sound like the same sound
played at another pitch, rather than faster-and-therefore-higher-pitch.
But on the genesis, this smooth perfectly-changing sound will be
something programmed into it, rather than recorded sound, and will not
sound much like a piano.