I agree with Jim that you probably don't want a road bike (curved handlebars). You can't (or shouldn't) ride a road bike on dirt. You can, however, ride mountain, cyclocross, and hybrid bikes on both pavement and dirt.
That being said, Jim has a good point with the cyclocross. If you can't find a cyclocross bike, however, you can go with either the mountain bike or the hybrid. If you want to change up the tires on either of those bikes, you can buy slicks. Slicks are tires with smoother tread, which allows the bike to move easier over pavement.
The hybrid bike will have you sitting up straighter, while the mountain and cyclocross bikes will have you more bent over. Sitting up is more comfortable, but being in a slightly bent position is more aerodynamic and will make it easier to pedal into the wind. Which for me, seems to be pretty much any direction I want go!
The bike sales person should spend some time with you answering your questions. If they try to rush you, say thanks for your time and go somewhere else.
"God gave you your body as a gift, so you should take care of it." - My Mom