I like the two Genesis Illusion games, but I'm not the biggest fan of the games around; they're good, but other games are better. Compared to the Magical Quest series, though, Castle of Illusion's easily better! The Magical Quest games have great graphics, but I've never liked the gameplay anywhere near as much as the visuals; I don't know, they just feel kind of easy and bland, sort of like another SNES Capcom Disney game, Aladdin. I get bored playing Magical Quest games. I have all three of the Magical Quest games on SNES, and they're fine, above average games, but Castle of Illusion is better where it matters -- better controls, gameplay, and level designs. Castle of Illusion's a good classic platformer that's well worth playing. Magical Quest looks better, but doesn't play better. Oh, and I like World of Illusion more than the first game; it's probably the best Mickey game on either console. (Mickey Mania is pretty good too, particularly on Sega CD, but the Illusion games probably have slightly better gameplay.)
As for the 8-bit Illusion games, the first two are alright, average to good games, but the third I don't like much at all. It's typical Aspect stuff, and I don't like how most of their GG games play. Even just looking at the first two I like the Genesis games more, but sure, they're good.
The SNES is a newer console than the Genesis, and while a case can be made for the Genesis to be as good as it in terms of hardware power, I think the SNES does clearly win out. I don't mean that in terms of game quality -- I like the two of them equally, they're both amazing -- and with its addons the Genesis arguably wins in power, but comparing just base consoles? Sure, the Genesis is a faster system with twice the CPU speed and a higher screen resolution, and yeah it has better sprite handling, but but the SNES's advantages, in its newer technology (more advanced chips), much larger color palette support, and something you don't mention but is quite important, transparency support, are pretty huge. Transparencies and color are a big deal! The SNES also can do fantastic parallax. When optimized properly SNES games can run plenty fast, too, and they often look better than Genesis games thanks to having more colors on screen and a much larger palette. And of course Mode 7 is a nice thing; you can get close to some aspects of Mode 7 with the Genesis, with clever programming, but it's not quite as good. Based only on the base consoles I do think the newer SNES wins out on graphics. As for sound, the SNES's sample-based sound chip was fairly new technology at the time, so it's a more advanced design than the simpler audio chip the Genesis uses. I do think the Genesis is usually better at some kinds of sound, such as techno-style electronic stuff (which I quite like), so choosing which is better is hard, but the SNES's is more technically advanced, at least.
Oh, and as for the Genesis's resolution advantage, it is quite real but whether it's important or not depends on the game. I mean, a game designed for the SNES isn't necesarially improved by going to the higher resolution of the Genesis. On the other hand, Genesis games ported to SNES often do lose something, as important things may be off screen...
It is tricky, though -- after all, one other thing about the SNES is that like the NES, it was designed for addon chips to be in carts to give games a boost over the base specs. The base SNES can do some really impressive things, but it does more with addon chips... but Sega took a different route, with games that almost never use addon chips but instead releasing hardware addons instead (Sega CD, 32X). The SNES did have one addon, the Satellaview, but that didn't have any graphical boosts in it, only streaming games and streaming live audio support. Comparing only base consoles, with no SNES addon chips, isn't really fair because the SNES was designed or addon chips and lots of games use them. But if that also counts hardware addons, the Sega CD and 32X added a lot to the Genesis, and their games can be pretty powerful -- both support hardware scaling and rotation on sprites, the 32X supports more colors than SNES, the 32X can do way more polygons than even the best SNES addon chip, etc.
So yeah, it's a tricky subject. They're so different it's not a question which will probably ever have a definite consensus answer.