Start with the mountain bike and see if the craving for a road bike is still there after some time. I am new to riding but have really enjoyed both. I was riding the mountain bike (2010 Giant Revel 1) on pavement at a local park for exercise. Thought that a road bike would be more reasonable so I went and got one (2011 Giant Defy 3). What a difference! I shaved so much time off of my 5 mile workout. I thought about selling the mountain bike to pay for the road bike but didn't. I'm glad now because I've found trails at the state park that's down the street that are amazing.
To each his own.