I have only used soft luggage in my RTW travels with a small aluminum topbox for my most valuable things. In over 4 years around the world I have never, I repeat never had anything stolen from my bike. In the last leg, NZ to Thailand I purchased a pacsafe. I used it seldomly, trusting my intuition. I however used my cable lock alot, locking my riding clothes and helmet etc with it.
You can almost always find a safe place to put your bags.
I have seen aluminum boxes that had been broken into on several occasions. It ain't pretty, and if you really value your things, you bring them with you anyway.
Whatever you choose, Your best defense is common sense.