The point isn't wether a soft or hard case is more durable, it is about what does the Law require. As explained to me by the Legal Defense Fund lawyer if impenetrability was the desired aim of the Law then you would have to hang 500 lb weights from your gun case to prevent theft, hard or soft case, and have it guarded by starving Bengal tigers.
The point of my call to the Legal Defense Fund was to understand if I might be charged if a CFO agent saw me at a match with a restricted in a soft case.....answer....good luck to the CFO. Read the Firearms Act, it is instructive. A container can be a paper bag. Sturdy would be made of something stronger than paper. The guidance given to me was that if an average person, without tools, could not quickly gain access to the contents of the bag or case it meets the requirement.
Call the Legal Defense Fund yourself to find out more about your specific situation.