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There are generally two types of six speed automatics: ones with torque convertors and ones with dual clutches, aka "manuamatics". Efficiency wise, the latter is more efficient because it is basically a manual transmission that shifts and clutches itself. Ford's small cars and a few VWs use manumatics (or DSG, as VW calls theirs). Toyotas still use torque converters. Usually the only way to tell what a car has is by driving it because sales literature isn't exactly aimed at the technically savvy.
Economy wise, CVTs are somewhere in the middle. The of their advantages is a much higher top ratio for highway driving. As an example, my brother in law's Mitsubishi Lancer 2.0L CVT is at 2000 rpm at 75 MPH, which is incredibly high gearing for such a small engine. A CVT has the advantage of being able to quickly and seamlessly downshift as soon as you want to accelerate from that speed.
In summary, the six speed autos and CVTs each have their strengths. I do my research solely based on fueleconomy.gov and would not hesitate to buy either type of transmission, as long as the EPA fuel economy numbers are good.
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