For getting around the city, I recommend you look around for an older road bike. You see a lot of these on the streets in cities not only because they are relatively cheaper, but they are also less likely to be stolen. As far as single speeds, I don't get it. My experience with SS is that you need to have proper gearing in relation to your weight/strength, which can be a pain. For example, with my friend's bike, he can fly on that single speed, but I can get on it and pedal like lightning, but it won't go nearly half speed. I'm a few inches taller than him, but also probably 60-70 pounds heavier. I'm much stronger, yet that means nothing since the gearing isn't high enough for me. Fixed gears don't really make sense either. There's a reason why track bikes are called track bikes and road bikes road bikes. You're boned when it comes to climbing with a fixie, but at least you'll build considerable strength.