Go to as many bike shops as you can find and ride every bike that is sized appropriately for you within your price range. Road bike, hybrid, mtn, it doesn't matter, as long as it fits. Buy the bike that is most comfortable for you as that will be the bike that you are most efficient on. Absolutely do not let the salesman sell you an inappropriate size just because it's in your price range and that is what he has available!!!!!

For your price range, my preference would be an intermediate level flatbar roadbike, or hybrid. An intermediate road bike (touring geometry) or more specifically, a cyclocross would be my first choice, but $600 probably ain't gonna buy a new one. Given what you say about your experience and apparent needs, it wouldn't take long for your personal performance to be beyond that of an entry level hybrid. A tighter wheelbase and less upright position will get you from A to B much faster than a true "urban" bicycle.