I'm unsure about VGM, but Zophar's Domain
has a good selection of GYMs.
Too bad there's not an emulated format like .SID or .NSF, since GYMs suck ass.
Also, Genesis wins for synth-style music, as it has 6 FM channels (each selectable for left or right channel, or both, IIRC), plus 3 PSG channels. If you use the DAC, you lose one FM channel. But Amiga wins for music using samples, since Paula is clearly much better than the Genny's DAC. Although, the Z80 made the DAC *better*, not worse, since you could dedicate the Z80 to just pumping samples to the DAC, while handling the FM+PSG with the 68k. Although many later games used the Z80 for all sound output, which lowered the quality of the DAC samples (for example, look at the samples in SSF2, which were absolute shite). You want to hear good Genny music? Go play Sonic 1, 2, 3, or 3&Knuckles.
Also, as far as RAM/ROM, Genesis wins over stock Amiga, with 4MB of ROM space available.
A decent direct comparison could be Mortal Kombat, since both Genny and Amiga versions were developed by Probe. Genesis MK had a smoother frame rate, and possibly slightly better graphics, as well as no load time. Plus, the images of the fighters on the menu screen actually scrolled on the Genny. But, the Amiga version's music was much better, although less varied (I absolutely LOVE the loading music, so much so that I don't really mind the load times). Plus, the Amiga version had the fighters' names actually in the health bar, as opposed to the Genesis kluge of putting it above the health bar.
In short, I feel the two machines are fairly equal, in general terms - Genesis wins in some areas, Amiga wins in others, and games on both were great.
One more thing - the Genesis could not do 64 colors in a single sprite - you have 4 palette lines, each with 16 colors (15 plus transparent). Every tile on the screen must select only one of these palette lines - to get more than 16 colors in a single 'area' on screen, you must overlay more than one layer of graphics (using sprites, or layer a+b). IIRC, Eternal Champions on Genny did some nifty tricks to get up to 512 colors simultaneously (half of which were due to shadow/hilight mode).