Is it better to get an entry-level carbon bike or a similarly priced aluminum bike?
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 tecniques learn laying up carboon, 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 ebd carbon frames, while having teh characteristics of stiffness and complance 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 compenets leads to a better bike for the same build cost. As carbon is consdidered more desireable by many, it fetches a higher price for the bike.  

TD;DNR when buying a bike, look at the whole bike, not just the frame.