Jenny, I usually use the Civita backpack, but 99.8% of the time I carry it on one shoulder like a purse. I, too, don't want anything to happen to my bag. It stays safe under my arm. Plus, I can put it on my back if I want/need to (when I don't want to take the chance of it swinging free - such as bending over frequently while reading plaques, shopping, etc.).
Either is as secure as the other one. Since there are at least two of you, you watch each other's back literally. When in very tight quarters such as packed metro cars, busy street markets, etc., put it under your arm like a shoulder purse or hold it in front of you. Keep an eye on your zippers. Keep a low profile when accessing your wallet. Keep the 'goodies' in the deepest and hardest-to-reach area of your bag.
Mainly, just be vigilant and watch each other's backs at all times.
Things that belong in a moneybelt are in my moneybelt.
But as Kent said, you've already answered your own question ;-)