As long as you are on the bike and keep your eyes open you have an advantage. If you get a puncture in a bad neighbourhood you are really exposed. Invest in the best kevlar protected tyres you can and practice quick repairs.
A shackle U lock is as good as anything else for defending yourself and it has the added advantage that you have to carry one and it is completely legal. You need quick access but anything you carry in your back pocket, you have to land on in a fall. I keep mine under a bunjie chord, on top of the rear rack, It gives me secure but instant acess for lockups as well as other contingencies.
I have used my whole bike as a defensive shield. Pick it up and hold it at chest level. If you hold it low, attackers will kick your gears but up high they just dont know what to do.