Metroid was first, Metroid is better.
Super Metroid > anything the Halo series can come up with.
...imho. I prefer the storyline in the Metroid series, as well, I find the games to be a bit more challenging than those of the Halo series (I do own Halo 2, and I did, at one point, have a copy of the original, as well) in that there's more to do than simply go from point A to point B, shooting everything in between. Again, that's my take. Halo never struck me as anything more than your quintessential FPS. Then again, these games (depending on which titles you're talking about) are pretty different. It's hard to draw parallels between the classic Metroid titles and Halo, and it's not really a perfect match to treat MP as an FPS that's designed along the same lines as Halo. However, I digress. A poll is a poll, heh.
EDIT - Oh, right. Moved to the VG section.