Does Halo 3 or Halo 2 have a better campaign?

I'm about to buy an Xbox 360 and I was wondering whether Halo 3 was worth it. I know that the multiplayer is the best part, but my internet doesn't reach where I'll be playing. I've been looking at some halo 2 campaign videos, and it looks really good. So my question, should I buy Halo 2 or Halo 3 (I'll only be playing the campaign). Please give details. I want to know what the terrain/landscape is like in both. I don't like the inside levels (like some of them in Halo 1). Please let me know...