Classic has a short height, black eyes, a potbelly, 6 spikes, etc.
He is the classic version of Sonic from the Genesis days.
Modern has a taller height, longer legs, green eyes, soles on shoes, etc.
He is the modern version of Sonic introduced on the Dreamcast, and so on.
First Appearance: Sonic the Hedgehog
Moves: Spin Dash, Spin Jump/Attack, Sonic Boost, Sonic Drift, Slide, Light Speed Dash, Stomp, Wall Jump
Transformation: Super Sonic
Forms: Modern, Classic