Er, Halo3 is more successful then both of those games combined, maybe even with their prequels.
The games you've probably meant are the Modern Warfare series, which are heavily scripted tunnel-shooters, that's where the true competition for Reach lies. Fortunately they don't have to go against each other this year (although Treyarch comes up with another COD episode, but I doubt it could get to the same level of sales).
I personally prefer the sandbox approach, so Reach is probably gonna be a full price purchase, while both MWs I only borrow from friends...
Your correct they also mention MW2. I know which I think I enjoy more, the Halo experience, but I have clocked more hours online on MW2 than any Halo game to date, which I didn't believe until I checked. It is an interesting question though because develoeprs are constantly going on about bringing the film experience to games and in the main that leads to tighly scripted tunneled games.