16" with no emotional attachment or high tech gadgets. That way you can drag it around everyday and not worry about legalities, finish, function or the ever present risk of theft from a vehicle...
Someone correct me if I am wrong but isnt it illegal to have a loaded rifle in your vehicle in Indiana?
If that IS the case, I would go with an AR Pistol simply because you can keep it loaded legally everywhere you go.
I'd go with a 16", non pinned barrel. Carbine or middy, whatever floats your boat or whatever is cheaper. Im not a fan of pinned on permanently attached stuff. If it has to be 16", might as well take advantage of the reliability and ease of swapping parts.
I like short barrels, but I'd go 16" first. Buy quality parts and you will thank yourself down the road.