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: 
Essentially this is about low end carbon versus high end alloy. Carbon is typically lighter and less harsh (more vibration absorption) than alloy, but is not as robust. I doubt that the alloy is much heavier. I think it will come down to the difference in component quality and your budget.

Personally for a training bike I would probably go for the alloy, but that is driven by my financial situation. If the bikes are of similar price and the components are of the same quality, I would consider the carbon. However I would not trade component quality for a carbon frame.

Edit : Seven years after the answer was give.
Many of the techniques learn laying up carbon, and with advances in hydroforming alloy, have lead to allow alloy frames that are significantly cheaper to fabricate yet nearly as light (if not lighter) than low end carbon frames, while having the characteristics of stiffness and compliance where wanted. You can no longer discuss 'Carbon' and 'Non-Carbon' frames as most major manufacturers have 2, 3 or more variations of carbon from 'cheap and cheerful' to 'insanely light and expensive'. The cost saving in Alloy frames put into better components leads to a better bike for the same build cost. As carbon is considered more desirable by many, it fetches a higher price for the bike.

TL;DR when buying a bike, look at the whole bike, not just the frame.
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!