Pretty tough call, if you ask me. The city has far more dangers, but they are constant dangers that rarely hide from you. In the suburbs the dangers are fewer, but faster and less predictable. The two require different skills and different strategies. The one you're less familiar with will be more dangerous. In my own case... I can't make the call.
I'm pretty familiar with both. In general, I'd rather have a narrow, twisty or tight feeling road if I'm on a bike. I want car drivers to be uncertain, a bit nervous or alert. Doesn't really matter if it's far out in the country, or in the middle of the city, if the driver is in full defensive mode, I am much better off.
Next favorite is medium size arterials. Stuff like a 3 or 4 lane street, where there's plenty of room to pass, but it's also only about 35-45mph (I don't care what speed limit it's posted at... I care what speed drivers are actually going). Then even if there's not officially a bike lane, I can take a lane, and drivers can get around me. These sorts of roads tend to be *very* busy and have very unpredictable traffic.
Big arterials or small highways suck. Once drivers think they have a "right" to be going over about 45mph, things can get really bad, really fast for me. I still use these sorts of roads, but it was really helpful to work on foot or on city buses before I started riding on some of the local ones. Taking the time to pay attention to how traffic flowed made it a lot easier.