Commuter Weekend Bike?

Usually I drive everywhere, but I really want to get a bike to use on weekends to run errands and for leisure riding so I can save on gas. I would mainly use this bike on the pavement, but there are a couple of dirt trails near me that I would like to try. They're very basic; no jumps or harsh terrain, just pretty flat dry sandy ground. I was looking at either the Schwinn World GS or the Schwinn World Avenue/Street or even the Kona Smoke. What do you think of these bikes? Are they good for my use and are they even good bikes? Or would you suggest something else? My budget is $500, but I can go to $600 or even $650 if I need to. So something in that range. Thanks for your help!