Two things regarding the difficulties:

1. The only difference I noticed between all 3 difficulties is that the bosses take more hits to defeat. The rest of the levels are exactly the same.
2. The other thing that's different is that harder difficulties have more continues. This means they are actually faster since you don't have to be so cautious about losing lives, since you have enough continues to retry the boss or the levels with all 9 lives and keep going.