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: 
From <http://www.livestrong.com/article/86919-aluminum-vs.-carbon-bikes/> (I can't testify to the accuracy of this)

> 
> Aluminum frames possess the shortest fatigue life of any material used
>  to manufacture bicycle frames. The typical aluminum frame possesses a
>  life expectancy of five to 10 years. Conversely, carbon possesses the
>  longest fatigue life. Most manufacturers provide a lifetime warranty
>  on carbon frames. Some manufacturers offer a lifetime warranty on
>  their aluminum frames, but most offer five to 10 years.
> 
> 
>
This is a very interesting question. Aluminum vs Carbon. Will it be long before the two are in the same price range - on it's way in the very near future. I have an Aluminum and a Carbon frame road bike. The aluminum bike has a longer wheelbase, more relaxed seat angle (72.5 vs 74 degrees) and square formed stays, and deflects most of the bumps and vibration almost as well as the CF frame. 

I'd say geometry, and even more, exact fit, within 1cm or less, are slightly more important than material for comfort, efficiency and handling. 

Calculate your frame sized based on Steve Potts bike fit theory. If you are 5'5 or shorter, divide your height in cm by 3.3, 5'6 to 5'10 divide by 3.2, and taller, divide by 3.1 to get your correct frame size. It works.