I can't agree with that. I'd say at any one point, for me at least, I want 2 styles of FPS to keep me interested. Halo's slick arcade feel is totally win, at the same time KZ is a lot more weighty, maybe a bit more tactical and slower paced. Well, it isn't tactical but you get the point that they are both unique to each other in the online experiences.
Sometimes I'm in the mood for one, the next I'm in the mood for the other. I wouldn't say gameplay really has much to do with it. The map designs are better on Halo, Bungie have done well in that regard, but you'll get the same level of fun from either title.