For city riding within the confines of traffic and all the hazards that go with it, I would suggest you stick with whatever bike feels most stable to you. I've got a road bike that I like to take out for long cruises with little traffic. But if I take that bike out into stop/start traffic, I just don't have the confidence factor that would make it a safe choice. Its drop bars and tuck position make visibility more difficult than a more upright ride IMHO. To me a road bike is best suited for the open road whereas an ATB is more appropriate for heavy traffic.

In Europe and Asia where cycling is considered more a form of transportation than the US etc., commuters are actually another named classification of bike. They have wider tires and step through frames with riser bars for an upright ride. They're built for safety, durability and stability of ride. If you look at them though, you wonder how they get anywhere at all. My compromise was to take my ATB with a step through frame and equip it with 26" x 1.25" invert slicks, road pedals and a lighter more streamlined saddle. It's incredible how much better the performance is. Next month I'm going for aero bars that I found which adapt to my risers. That will give me the option for the tuck. It's an alternative you may want to think about.