I used to say box every time after dealing with cheap bags, even air freighted 7 bikes to Oz in cardboard boxes all fine.
I now own an Evoc bag and the missus has one on order. The only thing is I do fly with my bike about 8-10 times per year and haul it through a lot of airports.
It’s much better than a box for carrying, moving and protecting a bike. The wheels provide the vertical bracing (ignore them when they tell you to let the tyres down) the rest of the bag holds the bike in the right place.
Remove rotors & bars, use frame spacers and take pedals off, I find old towels and pillow cases are quite good put anything loose in your camelback (tools etc.) and do it up.
Check your airline weight restriction for sporting goods. If you have a 23kg limit then you will struggle to put too much extra in. Unless you have a rigid hard case helmet is hand luggage (a hat).