Opinions on this probably vary quite a lot, and the answer probably depends a lot on the specifics of each alternative. But, I would tend to think that living close in to the city will tend to have a lower environmental impact overall. In the city, jobs and shopping are likely to be a lot closer to your home, making walking, cycling, and transit viable transportation options for more of your trips. You'll probably live in a smaller home, which took less materials to build, and which takes less energy to light and heat. All the infrastructure, like sewers, roads, powerlines, etc. are much more compact and more heavily utilized.

If you want to reduce your evironmental impact even further, I recommend buying food from a local farmer's market or CSA when possible. The food travels a lot less far, is often organic, and I find it to be fresher and tastier than what I can get from the supermarket. It does require a bit more creativity and flexibility though when you have to buy fruits and vegetables in season, rather that getting all your favorites flown in year round.

Another thing that you can do is to eat less meat, especially beef. It requires a lot more energy to grow a pound of beef than a pound of grain, and the industrialized meat production industry has large negative impacts on air quality, water quality, and public health.