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: 
Personally id rather have top quality aluminium than cheap entry level carbon. 

Whatever you choose, don't go daft with the upgrades-save your money and get a really nice carbon number in a year or two. 

Entry level bikes are great when starting out. You can strip them, fit new parts by yourself, which obviously is a great thing to learn, but if you dive right in and buy an expensive bike you would be very hesitant to start working on it yourself. 

Also, if you join a club and keep getting dropped then its only to be expected. Turn up with an s-works with di2 dura ace however, then not only would you feel a bit of a plonker getting beat by old guys on old raleighs, but I'm sure they'd also take great pleasure from it! 

Whatever you choose/chose, good luck and happy cycling!
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...