if you're going to be riding around your block for exercise...don't get one of those heavy bikes with front and rear suspension. it'll do nothing but slow you down and then the shocks will wear out before you know it and then you'll be paying to fix something you don't need.
goto a bike shop and find a decent used schwinn or something that's meant for road/hybrid use. it'll be easy for $100 or less and you'll end up paying for quality components and a decent frame instead of flashy looks and useless/heavy suspension.
also, the tech. at the bike shop should be able to help you select a bike with a frame that fits your body size and leg length well and will help you set it up properly be more comfortable and avoid injury. the last thing you need before going into service is to be riding around on an improperly-sized and setup bike and end up hurting your knees or back.