I would use carsdirect.com only from personal experience and from that of a close friend. I cannot comment on autobytel.com as I have never used them before.
How they nmake their money though is through sheer volume. The on-line seller (carsdirect.com) gets vehicles from fleet/manufactures for invoice (perhaps slightly less?) and makes their money on a slight mark-up. You the purchaser still pay above invoice, but less than MSRP. Carsdirect.com makes their cash on the mark-up and the volume of cars sold.
Remember MSRP is a HUGE mark-up that the dealer places on the vehcles so that when he gives you a cost say a few hundred bucks less, you feel like you have gotten a good deal. The truth is that only suckers pay MSRP. And if you are a good negotiator, you should be able to drive away from a dealership with only paying a slight percentage (read as $100-200)above invoice as a equitable deal. To negotiate a good deal, plan on being at the dealership for 4-6 hours, more if you are going to finance through the dealership, which is ALWAYS a bad idea! Remember, the dealer will use ALL possible means to squeeze every fraction of a penny out of you at your expense!!
Here is where carsdirect.com is beneficial. Their price will usually be just slightly higher than the vehicle's invoice. This can be ascertained by checking www.edmunds.com for the vehicle invoice price, (also check edmunds.com for the vehicle's True Market Value TMV). make sure you check the edmunds prices with the options you desire to get a true comparison.
Where I find carsdirect.com to be the best? Its for multiple reasons.
Reason 1. I DO NOT have to deal with car salesmen! Read some of the info at edmunds.com regarding dealership tactics to wear down, bully, and generally screw buyers. You will never buy from a dealer again.
Reason 2. My time is more valuable to me than the 4-6 hours I will be spending at the dealership purchasing a new car! I like to have my new car delivered to my house which is an option with carsdirect.com.
Reason 3. Did I mention that I don't have to deal with car salesmen and their shifty "Hi I'm your best pal" sales manager.
Reason 4. I don't have someone pressuring me to lease a car or trying to pressure me to "up-grade" to the LX or Limted or whatever crap I dont need edition of the car I want and can afford.
P.S. I have purchased over 5 new cars in my time, so i do have some experience at this. :)
Go visit Edmunds.com and absorb all you can from this site, it will save you thousands of dollars, as well as your precious time and a big headache.
Good luck!! And remember, buying a new car should be a fun experience, not an exhausting one!