The similarites are that they are both quadripligic's, They aren't happy with their lives, The wrote memoirs about their accidents, The have people trying to help them, People who love them but don't know what to do.
The differences are that in The Diving Bell and The Butterfly he can only blink, and in The Sea Inside he can move his whole head. In Diving Bell and The Butterfly he loses love, but in The Sea Inside he finds he can actually have love.
To me, The Diving Bell and The Butterfly is the more powerful one because it talks about hoe he can't communicate or talk or tellanyone how he feels. He can't describe it and he has no choice living the way he does.