I can give a bit of feedback as I own a 2010 Impala and rented a 2013 for a week about a month ago when mine was in the shop.
To start, I would look at other brands makes too. If you are stuck on Chevy's, the new Malibu's are nice and almost the same size, but are put together much better.
My 2010 has about 70K miles on it, this is the most maintenance intense car I have ever owned. I am a car guy and take care of my stuff, maintenance is always done on time. Runs great and is a really smooth car but lots of little things going wrong. I'm on my third set of front brake rotors (keep warping,Impalas are known for this) the transmission cooler lines are leaking, rear main seal leaking, power steering pump starting to make noise ( yes the fluid level is correct) ,had bad park interlock wiring I had to redo as it wouldn't shift into park or release the key from the column, passenger tie rod or ball joint starting to make clunking noise.... Never had a newer model car with so many issues. But, I do love the way it drives, very smooth/solid drivetrain and pretty good on gas, we get 24MPG overall. The interior is junk, all plastic and feels like it's from the early 2000's.
The 2013 I rented with the 3.6 felt put together much better overall,only had 6K miles on it though and from what I recall mine felt like that when it was new too. It was just as smooth as my Impala however, you have to rev the [censored] out of that motor to get any power from it, feels like it has no torque down low. Rev it up into the 4-6K RPM rev range and it moves, but it's annoying to have to romp on it anytime you want to get anywhere. Gas mileage was about the same, ride was the same,same junky interior, brakes felt a little better so i'm hoping they finally fixed them in 2013.
Hope that helps some. I'd go test drive them to see which you like more. Better yet, go drive a new Ford Fusion or Taurus. I just purchased a new Ford as IMO they are leaps above Chev in quality, tech and performance now. The new Chevy's look nice too but with all the issues I have had with the Impala just couldn't buy another.