To me it looks like you are talking about the game only, not the story. Here's what I think.
Halo 3 was a bigger, more loved game overall by Halo fans. It had an unforgettable campaign and a new multiplayer experience. It was definitely something to remember. Halo: Reach on the other hand, was a completely new experience for Halo in general. AAs, many more forge options, older storyline, newer characters. It was also Bungie's final Halo game, meaning it held a high place for sentimental value. It was what Bungie had always wanted to make and was their final effort on Halo.
Now gameplay-wise, Halo: Reach was more advanced and had a smoother multiplayer experience. It had a wider variety of weapons, abilities, and vehicles to use than Halo 3. Halo 3's multiplayer had a nostalgic effect on me, first time playing it I was so hyped. Halo 3 without a doubt had a better ranking system. The Armour in halo 3 also felt more valuable compared to Halo: Reach for me, most likely because there weren't as many sets and earning each piece was a satisfying and sometimes challenging accomplishment. Halo 3's campaign had an amazing storyline that continued the Master Chief's story and left us with a stunning cliff-hanger. Halo: Reach's campaign was definitely interesting and backed us up on Reach's story, which is always great for our lore knowledge.
I don't think either game is really 'better' than the other. I can't make a final decision because they both balance each other out with better factors than the other. If I had to make a choice between the two, I may end up saying Halo 3 because of my enjoyment whilst playing it, although I had ended up playing Reach more.