Tri Bike vs Road Bike

A road bike is more comfortable and widely accepted in group ride settings.  But if you plan to train for tri's specifically, and typically will be training with triatheletes only, then go for a tri rig.  In group rides with 'roadies', they dont like that while in the aero position on a tri bike, you dont have breaks (just bar-end shifters).  Triathletes dont typically fuss about it.

The other option is going with a road bike and buying clip-in areo bars.  Then you can always remove the aerobars if the situation warrants it.  But, the frame geometry is different between a road bike and a tri bike (seat post angle - tri bikes is steeper, like 78deg vs a road bike at 73deg), which means your areo postion is not as efficient with a clip on bar on a road bike and you typically become to stretched out while in aero (thats the whole road bike more comfy, 73deg seat tube condition).

I road a road bike for the first 4 years of tri's, then I got a tri bike.  I love the tri bike.  And I am significantly fast on a tri bike too.  So thats may $0.02.

Jon makes some great points with the group rides with a road bike versus individual rides with a tri bike. Personally, I knew I wanted to do only tri's when I bought my bike, so I've never regretted buying a tri bike over a road bike.

When I used to ride a road bike for 30+ miles, my hands would go numb from all the pressure on my wrists and my triceps would get sore, so I love having the aero bars from a purely comfort perspective. Also, I got a lot faster with the same effort as soon as I got my tri bike, partly because of the aerodynamic advantage, partly because it's a better bike, and partly because I had a better fit on the new bike.

Tri bikes and road bikes are basically different animals.

A tri bike does one thing really well: it goes really fast in a straight line on a flattish course in a race where you're not allowed to draft another competitor. Most of them don't climb particularly well, none of them handle particularly well and even the world's best rider would be a menace on one in a criterium or mass- start road race because you can't brake or corner or accelerate quick enough.

It's like a track bike ... it does one thing well and more or less everything else, not at all.

A road bike on the other hand is a generalist. It's nowhere near as good for time- trialling as a tri bike but it climbs, handles, stops (because your hands are much nearer the brakes) and accelerates better so you're safer and faster in a group.

I tend to disagree about comfort: a tri bike can be equally as comfortable as a road bike. The issue is that a tri bike is designed to be ridden in the aero position and it's difficult to find another comfortable position on one. Most people struggle to stay in the aero position for a sprint distance tri let alone an Ironman. On a road bike it's much easier to move around: most people ride most of the time with their hands on the hoods but, can switch to riding with their hands on the drops or on the tops to give their butt a chance on longer rides.

Wow, thanks for the feedback everyone, it was very helpful. So I guess I need both huh?  Maybe I'll start out with a road bike for now, and then consider a tri bike down the road. Hm.

I bought a Tri bike last fall and it has taken me a lot of time on the trainer this winter to get comfortable on the bike.  I was scared to death to ride it on the road to start, but now I love it.  Roads still make me a little nervous, but I am feeling confident on the trail system.
Others have made great points, a tri bike goes straight and fast, it takes some time to get use to the tri position, but once you get use to it and comfortable you will enjoy it. The huge advantage I see to the tri bike is it is soooo much faster than the entry level road bike I owned prior.  From my experience, there is a learning curve to the Tri bike, but at this stage I am loving it. 

Get a good road bike and add some clip on aero bars. I would look for more 'sport' oriented models if you are just starting out and not looking to get into serious racing. Example, the Giant OCR is more of a sport geometry which is still a good bike but it will be more comfortable for more types of riding than more aggressive geometries. I ride a Giant TCR, and for a tri set up I move the seat up a little closer and I have clip on aero bars. I know some people say a tri bike is faster but my bike is much faster than the rider is capable of pushing it . So that is my $0.02 and my road bike serves as a good tri bike and the clip on aero bars always stay of in case I get tired or want an alternative riding position.