Bag for me as I find boxes too big, ungainly and generally to cumbersome to transport even in a Range Rover.. I am however very fussy when it comes to bags as a large amount of them are next to useless in the protection stakes
I only use Aim drag bags for me rifles, available in a few different sizes and the quality /protection they offer and longevity is second to none
I do however store my Air rifles in hard cases as there is no room and generally not enough space in my gun cabinets