I'll put a vote in for the Chrome bag. I've been using a Chrome Metropolis for about 3 years - fantastic bag, and I'd never go back to a backpack. Never had any issues with it sliding around, and even with a heavy load in it, it's not a problem on my shoulder. Most of the weight sits on your spine, which is far more comfortable than a backpack pulling your shoulders back. It's fully waterproof too. One thing I've grown to really like is that you can easily spin it around to the front to get stuff out (keys, wallet etc), then just spin it back around on to your back. The stabilizer strap on the Chrome bag connects in a way that does stop the bag sliding, but I've found I only need to use it with really big loads; when it's just my commuting stuff in the bag, it stays in place perfectly without it.
For really heavy loads, I still use panniers (Ortlieb or Arkel).
If you do go backpack, a few of my friends swear by the Deuter ones.
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