Hmm....
Halo 3: Better campaign, characters, and story line. It was practically a single player co-op between Chief and Arby, and there's a bigger variety of enemies and vehicles. Before Halo 4. More connected to other games. BIG cliffhanger.
Halo Reach: Customizable Character (Noble 6) which is shown throughout the story. A more emotional story, with more characters/friends. People die (Yes, in my opinion that's a good thing, more believable and human). Noble 6 (Player) dies.
I think overall Halo 3 is better because it's more related to the whole Halo trilogy, considering it's in the middle of the games.