Classic is what Sonic would have looked like if he was put in Smash around 1998 or so. And it's his original design of what he looked like in Sonic 1 and 2. Classic Sonic features no voice lines and has sounds from the Sonic games on Sega like the spring sound and his Spin Attack sound on Sonic 2.
I dont think theres any difference between modern and classic rather than just being a reskin
Classic is what Sonic would have looked like if he was put in Smash around 1998 or so. And it's his original design of what he looked like in Sonic 1 and 2. Classic Sonic features no voice lines and has sounds from the Sonic games on Sega like the spring sound and his Spin Attack sound on Sonic 2.
Not to mention classic has Sonic's jumping sound from Genesis. Modern Sonic is what Sonic would be in the present day and he has a voice.