Didn't only Japamese Mark III games use the FM synthesis chip, kinda like NES didn't get expansion audio? And like Famicom, Mark III games have a different pinout. I'm not enough into SMS to buy a stand alone console that I'll barely use, but if US/EU games play on an unmodded Genesis 1 without issue the same as they would on an unmodded SMS, then I'll just stick to Genesis with an adapter. Power Base Mini looks a lot cleaner and cheaper than the rare and bulky Power Base Converters.
Then again, I finally got an AV Famicom because I was sick of dealing with fugly adapters in my Toaster, not to mention the occasional oddly shaped carts that didn't fit, forcing me to use the old Yobo clone with swapped duty cycles. I already had a nice collection of Fami carts before I ever got it though.