Superstar is uber overated... but anyway, Amazing Mirror do is MUCH better than Nightmare in Dreamland.
Nightmare in Dreamland is a remake of Kirby Adventure, one of the easier and (in my opinion) inferior Kirby games.
Amazing Mirror is MUCH harder and longer. It also use an unique level structure ; there are no levels, unstead the world is a bit like in the Metroid and post-Symphony of the Night Castlevania titles (read: there is only one gigantic level and you need to use several abilities to access every part and beat all the bosses, thus unlocking the door leading to the final area where the final bosses awaits). The abilities are well-made with advanced techniques without being as overpowered as they were in Kirby Superstar. The computer players are actually put to good use ; many puzzles require their presence although they are useless for important fights. Also, there is a minor element of surprise regarding who is truly behind all of the Mirror World's invasion...
Anyway, if you can, try out the game. It's well-worth it, in my opinion.
