What is the story about 'double clutch' on the newer cars? What are the advantages?

Two clutch designs disconnect both ends of the gearbox, both the engine and drive wheels are disengaged. This allows the gears to be shifted faster, and drive power applies again much sooner.

So a computer controlled dual clutch "manual" gearbox can shift in about 1/10 of second, including the clutch action.

I say "manual" in quotes because the driver isn't using the clutch or a direct gear lever, just pushing a button or paddle on the steering wheel and telling the computer to change gear.

This is one reason a Nissan GTR can get to 60 mph in under 3 seconds. Sheer HP and 4 wheel drive help too, but without the fancy clutches and gearbox it would probably take a second more.

Super fast automatic shifting, since the clutches alternate from gear to gear -- faster than a human can shift a manual transmission.

Go with automatic more expensive but more reliable! Gas engines < all the rest.