Perhaps over 30 years too late but I am definitely in the SNES camp in the SNES vs megadrive argument.
Reasons? Have you tried Megadrive gaming?
I've owned both and im still 50/50 SNES Megadrive. Some of the games for SNES are fantastic - Mario, Mario Kart, FZero, Donkey kong country but the Megadrive has a much more varied selection of game and does arcade ports better.
I have a Megadrive hooked up at the moment in the lounge.
SNES games do look more colourful and 'nice' but the Megadrive with it's more powerful hardware pull off some fantastic arcade ports - Virtual Racing and Virtua Fighter 2 as good examples. It also handles parallax scrolling much better hence more games like Sonic, Shinobi and Shadow of the Beast
I was an Amiga kid too but friends had Megadrives and SNES'. I didnt get a SNES myself until I was 19, Megadrive a few years later. They hold their value well today if you buy and sell at the right price
Back on topic though, a 3 year old gaming?? I do think Mario games would be good for young kids but 3 is probably too young