I think both are equally good.
I've loved Halo since Combat Evolved.
How can I not love some good alien blasting fun?
Gears of War is a great game for some of the same reasons.
Both have the same basics to them.
Common Goal? Save Humanity.
General Gameplay? Kill a crapload of Aliens in your way from point A to point B.
Tools of the Trade? Guns. Little Guns, Big Guns, Big Flipping Guns, and various melee weapons for getting dirty.
Gears of War is 3rd person and is a little slower paced than Halo.
Halo is a 1st person and its maps are generally very open.
Both are great games in general. If I had to choose between them?
Single player: Gears of War.
Multiplayer: Halo