Late post is late, but...
Personally, I prefer Gears almost 1000x to Halo 3. Granted, I'm not really a big shooter or Halo fan, but I find that Gears is just a lot more fun and easier to play (for me at least, the design and function of Halo didn't do it for me).
I like the more strategic feel of the game, because for me I can't get behind something that's ridiculously fast-paced and requires you to be almost brain-dead in terms of how fast your reflexes have to be. Then again, this is all coming from someone who didn't really play shooters up until after the release of Gears and Halo 3.
Halo, I won't deny, is a good game. Not the game I would choose to play or fangirl over, but a good game with a decent story. Gears for me is also a good game, one that I enjoy a helluva lot more than Halo 3 simply because Halo 3 made me feel like too much of a noob to really enjoy it at all. Gears is the shooter I choose to fangirl over because I freaking LOVE Augustus Cole. And I find it easier to relate to super-muscled no-neck humans with some feelings than those ultra-cool Spartans.
It's apples and oranges ultimately, though. No reason you can't enjoy both unless of course you have an allergy.