Depends what you need it for. If it's for fighting off tyrannical oppressors at longer distances, go for the long barrel. It will give you better accuracy at long range. Slightly harder to handle depending on what your requirements are.
For indoor uses, if you need this system, def go for the carbine, a shorter barrel is easier to maneuver.
I live in a crappy Kalifornia so our gun laws blow here. Consider that the ARs have swappable receivers, and are flexible.
Check out the last post on that site. Make sure to check the laws on barrel length for legality and stuff, I'm not a lawyer yet so I can't give advice on that. You will get the flexibility of other calibers too.