There are many differences between Akuma and Shin Akuma. Akuma and Shin Akuma are both characters in a video game. The video game is about street fighters and there a lot of series in the game. One notable difference between the two is that, the video game was initially about Akuma before Shin Akuma was created as the second character in the video game.
The new form of anything is always seen and considered as a better and stronger version of it. This is the exact case with Akuma and Shin Akuma, Shin Akuma is considered to be stronger than Akuma. Akuma is very slow in action and speed when compared with Shin Akuma who is very fast in speed and also very fast to act. However, many still consider Akuma to be a very strong man just that he's not totally using his power, unlike Shin Akuma who practically uses all his power over rivals.