Unless you are planning on doing a lot of drag racing, I'd chose a 2011 GT over a 2008 GT500. You'll really want an '11 GT if it is going to be a daily driver. Everything about the '11 is so much more refined than the 05-09 Mustangs. The clutch/transmission are much lighter and smoother, the ride is better, the handling is better, and the chassis feels better balanced, the traction off the line is better, the interiors are nicer, have more features, and are quieter, and the fuel economy is better as well.
The only thing the 2011 GT doesn't do better than a 2008 GT500 is straight line speed, and even there, the GT500 doesn't win by as much as you may think. It's only 88hp more, and the extra weight negates some of that.
The 2013 GT500 is truely going to be a beast. Unfortunately, it is supposedly going to be MUCH more expensinve than the current GT500. I doubt you'll be able to get a used one for anywhere near a new GT, even 3 or 4 years out.