Well, even though they both have the same story, the game play is actually entirely different. Zero Mission is fairly easy (except for a lot of the item pick-ups), but it still has that classic Metroid feel and some great bosses. It's a great game.
Then again, so is the original, but it's completely different. Aside from similar geography, the games share very little except story. The original Metroid is extremely non-linear, and very few items are mandatory to complete the game. You can go through the game in any order you want, with almost any combination of equipment. It has enormous replay value for this reason alone. It's extremely challenging, and it has an extremely great sense of atmosphere, because you hardly ever know where you have to go next or what to do, which lines up very well with being in an underground base.
Both of these games are excellent, and I wholeheartedly recommend you get both, because they really don't feel like the same game. If you beat Zero Mission, you actually unlock the original Metroid to be playable on the Zero Mission cartridge. If you buy Zero Mission, you could play both in this regard.