The downside of the 6 person version is that it probably isn't as wind resistant. Same poles, but a much larger profile. The 4 person version is really solid. Also, I think the 6P tent would be too large for some of the sites where I camp. Add it in the vestibules and it's a very large tent. I think it's big enough that it would be a struggle for one person to put up. Big advantage, I guess, is that you can fully stand up in the 6P.
All things considered, I think the Halo 4P is a perfect compromise for 2 person car camping, even in pretty bad weather. I've only seen one reference to the Halo 4 failing in wind: thunderstorm with hail and 60 mph wind. Tent held up OK until they unzipped the door and the wind got inside the vestibule and collapsed the tent. But, I'm not going car camping if the forecast includes storms with 60 mph wind. Or, if I did, I'd take my 4 season tent. :)