Honestly I prefer the sound of Genesis/MD because it's aged slightly better. A lot of music on the SNES sounds very muffled, the Genesis pulls off some really clean synth sound to this day but regardless both of them have blessed my ears with some sweet tunes. In the hands of a good audio programmer/composer/technician, it doesn't matter which chip is better but rather how well someone can utilise the tools they have.
I mean, on Genesis you can get masterpieces like
this, but also get steaming turds like
this.
While on the SNES you can get really great songs like
this, but still have someone managing to make
this awfulness.
Maybe if the SNES sound chip had more than a measly 64KB of memory and had a few more channels then yeah, it would of been undoubtedly a better chip.