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: 
I was faced with a similar situation about two years ago. I decided to go 2nd hand aluminum. The reason was that I didn't want to spend too much before I really knew what I wanted. There are several different types of road bicycle set-ups. 
This is what I didn't know:
 Will I want to ride long distances (100+ miles) or sprint races (30-40 miles) or just regular rides with friends (a couple of hours here and there). Each of these bikes would have a different set-up, material and geometry. 
 Will I ride a lot of hills? Would a compact crank or regular crank be better to my riding style?
Buy a used, but decent condition al bike. After 2000km, you'll have a good idea of what type of bicycle you want to spend real money on, and you'll be able to sell the current one for very little loss. If you don't sell it, then you have a winter trainer for use inside. And in the all too familiar case that you buy the bike and rarely use it, you didn't spend 1500 to 2000 on something that just takes up room in your garage.
Quality-wise bikes are a lot like chains. The total quality of a bike (where with quality I mean stuff like durability, smoothness, ease of use, ease of maintenance, etc.) is very dependent on the weakest link.

An expensive frame with cheap components makes for a worse bike than a mid-range frame with mid-range components. So given the choice I'd go for the (relatively) cheaper frame with the (relatively) more expensive components, all dependend on what I actually wanted to spend on the bike.

There are "ride quality" differences between carbon and aluminium and steel. Carbon is usually less stiff, so the ride is a bit more comfortable. Steel has the same properties as carbon and is more durable, aliminium is usually a lot stiffer and hence the ride is harsher. For that reason you get aluminium frames with carbon fork and stays. The main disadvantage of steel is of course weight, but you have to look at this critically. The 0.5-1 kg weight reduction you get by going from a really good and durable steel to a carbon frame disappears in the 15-20 kg of fat you are lugging around anyway, and unless you plan to go ride a lot of serious races you will never notcie the small gain of the lighter frame.

Where I'm going with this: In the standard bikes you buy the frame is often of higher quality (and price) compared to the other components. You can get a better bike for less money if you pay attention to this when buying your bike. Spending some time talking this over with you friendly LBS owner will help here.