Metroid Fusion - You're once again in Samus' boots. This time, you're headed to a research station that has suffered damage from a large explosion. You've been stripped of your abilities during a battle with a new lifeform, "X". (Pretty original name, huh?) The gameplay is exactly like that of the original Metroid, except much smoother and faster paced. The look is even the same 2D side scroller, except with better graphics. All of the power ups are here, bombs, missles, new armor, etc. The big difference between this game and zero mission is that this game is more linear. The storyline is constantly flowing via a computer "guide" that helps you navigate the station. You'll never go somewhere you shouldn't be. For this I've always thought the Metroid series was incredibly well designed as far as how the levels are laid out. Anyway, towards the end of the game, you'll be able to explore more freely by accessing areas you couldn't get to before. The only bad thing about the Metroid series is how short the games are, I finished this game in 6 hours and I wasn't rushing. In fact, they reward you with better endings (basically a picture) if you finish the game in certain times.
Metroid: Zero Mission - See everything I wrote above? Same thing here, expect this game isn't as linear and you don't have a "guide". I've heard people say this game is shorter than Fusion. They're full of shizzle. The games are the same length.
Basically, if you liked Metroid or the games in the series, you'll like these two games.
6 out of 10 for both games. They're a lot of fun and addictive, but I hate how short they are.