Zwei is a definite step up from its predecessor in all areas. The gameplay feels more refined and has a better progression as your dragon evolves. The story is more compelling (cutscenes between levels! voice-over narration!). The level design and bosses are far more inventive, with branching paths and exciting set-pieces. It's a lot more merciful too, giving you unlimited continues, a save system, and a special attack to help with clearing the screen faster. It's awesome.