It all depends on what you are looking to do with it. If this is an inclose weapon, you don't shoot past 100 yards, or mainly shoot 55 grain ammo, then the choice is as simple as heavy carbine or light carbine. The main reason for the 1:7 is heavier ammo, if you are one of the people who only shoots 55 and 62 grain, its not an issue.
The 6721 is working very well for a lot of people. So is the 6920.