Question: I am going to buy my first proper road bike, and I have been comparing entry-level carbon frames with top-level aluminum frames in the same series (Giant Defy). What are the benefits of carbon vs. aluminum frames? Is it better to get an aluminum bike with higher-end components or a similarly priced carbon bike with lower-end components?

Answer: 
Or, as I have done numerous times, buy a good used bike from a Local Bike Shop that has a good frame that you think you will enjoy long term, and reasonable components. Then you can ride, at a good price, with good stuff, and you can decide what to upgrade and when. Last time I bought a complete bike was 1999. I am riding 3 different bikes right now, a 1998 Merlin Ti, a 2004 Merlin Ti/Carbon and a 2008 Merlin Ti, none of which I paid more than $2300 for, and each are now worth a lot more due to upgrades over the years, on my schedule and budget.

All personal choice, but I've been pretty happy...
You are better off buying a Reynolds steel or chromoly steel road bike. Do not believe the hype about carbon fiber. You can buy a steel bicycle just as good or even better than the CF bicycle.