re stapled boxes, up until recently, I was lucky always to have access to stapled boxes, which I always figured was stronger and certainly easier to not have to shore them up myself.
On the return of my last trip, I only had a flattened box to use, so I bought extra tape and with the help of someone, taped the heck out of the bottom of the box, and when the bike was inside, taped the heck out of the entire box, wrapping tape around and aorund and around the entire box so it would be as strong as it could.
It arrived fine, and you know, I actually think that as there was so much tape all over the place going all around the box in every way possible, it probably was stronger than just a stapled one in case it got caught in the rain while being loaded.

Ive never used a bag, and have misgivings about that, but I do realize that its really down to the individual workers that day, and how they feel about life, the universe and everything--ie if they are in a pissy mood that day, its probably not good for our bikes, in a box or in a bag....

bottom line, if you go the flattened box route, buy at least one big roll of clear tape and go to town on it.

friends who travelled last year with bikes in bags said that when they went to pick their bikes up at a part of the airport, they saw that they had both fallen over and had suitcases kinda piled on or around them.
I dunno, I still think there's a lottery aspect of either way....