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.
You didn't say at first that you live next to a trail - that settles it - mountain bike. I might be a little biased because we have no safe place to ride road bikes and most folks around here have very little patience with our two wheeled sports taking up the road. If I lived where bikes are tolerated, I'd ride my road bike. For where I live, I ride my mtn bike much more than my road bike.
Get a very good mountain bike and a set of road tires. I used to commute and do some touring on a mtn. bike with "fat boy" tires on it and could keep up with 98% of the road riders, some were trying to drop me (lol, I was riding about 500 a week at the time). When money allows pick up a NICE road bike!
2008 Rubicon Unlimited
Jeep Green, Automatic
Currie 4" Rock Jock w/rear Anti-Rock
"special" Mopar winch bumper, Warn 9000i
Evo front and rear LCA skids, RROR full skids
Fischer 4x4 mfg tire carrier, evap relocated
37" MTR's on DIY beadlocks, and about 2 pages more crap
Kenwood TM-V71A, Cobra CB
i'll preface this with the fact that i have both...
road bike is definitely the easiest to just hop on and go if you are short on time and looking for a workout or to cover some miles
mtb is WAY more fun for sure, depending on how close you are to a trail. I agree that in the meantime a mtb will be more practical as you can use it for both until you get a road bike. you will find that one of 2 things will happen... 1) you'll ride your mtb on the road a few times, say screw it and go buy a road bike... or 2) you'll ride on the road once or twice, and then never again. haha
i suppose ignorance is bliss... once you ride a road bike, a mtb on the road REALLY sucks.
my personal opinion, and one that i'm sure others would echo... if you are serious about it, invest in a decent bike to begin with - scrape a few extra pennies and get a nice bike with good components. cheap bikes suck to ride, are expensive to keep running, and will eventually either turn you off of the sport, or you'll end up buying a nicer bike later
Niner EMD 29er
Kona Unit 29SS
Trek Carbon Road bike
Riding 20 miles on an expert mountain trail is like riding 50 miles on a road bike. Mountain biking is much more aggressive and a much better full body workout. Plus there's no traffic, you own the trail not the cars.
I've been riding a Gary Fisher Cake 3 for 8 years.
2008 Red Rock JKU X
285/70/R17 Duratracs with stock 3.73 gears
Tera Flex Leveling Kit
Spidertrax 1.5" Spacers
"As you slide down the banister of life, may the splinters never point in the wrong direction."