I definitely prefer bags with a zipper on top. I am very aware of pickpockets. I do have a few open top bags but I never carry them on the underground or other places where I know it will be crowded.
I am especially careful when on vacation in certain European cities, they tend to seek out tourists.
My friend was on a weekend holiday in Austria recently, carrying her Pallas Clutch (zipper on top). Suddenly 3 girls surrounded her on the street, asking for directions. A few seconds later they all ran in separate directions and her wallet fell to the ground. They had opened the zipper, taken the wallet out of her bag, then taken the cash from the wallet - all in about 10 seconds without her noticing anything! They only took cash from the wallet, none of her cards...maybe they did not want anything traced back to them...? Her wallet was also LV and worth more than the few Euros they took...