I agree with *nearly* everything that Flanders said, except for the "if you don't care about the horsepower difference go with the older model."
The 2010 is a markedly different-acting car than the 2011. Yes, if you get the Premium V6 coupe the creature comforts and the interior design share a lot of similarities. But under the hood, the 2011 is a totally different animal. Besides changing the engine, Ford also modified the suspension. They took some of the suspension goodies from the 2010 GT model and applied them to the 2011 V6
model, to compensate for the horsepower and torque gains.
The difference is a much more composed car, easier to control. When I rented a 2010 V6, it drove and slid and acted like an old-school Mustang - rather unrefined and fickle when pushed to its limits. The 2011 V6 is a lot easier to push to its limits. I like the suspension feedback much better.