Difference between Shin Akuma and normal Akuma is that Shin Akuma is faster, can chain moves together quicker, deals out more damage, has a double air fireball, faster teleport and his death touch special is faster and goes alot further.

I've managed a 13hit combo (normal moves chained together not a shadow move.) with Shin Akuma in SFA3 which you can't even get close to with anyone else.