What is your purpose for the car?
For the money, NEW I would get a 2011 GT500 with the track pack. If you can drive they will go 11's stock. They have made a LOT of improvements to the GT500 for '11 like aluminum block, suspension, gears, etc. that make the car really handle if you do more than drag. I believe it's actually lapped several tracks quicker than the Z06 as well.
Then again, the Z06 is a great car and they are amazing. It just depends what your plan is? If you want looks or something rare, I wouldn't really get either. Hell, in Houston you see Vett's and Z06's like Honda Accords....they are EVERYWHERE! GT500's you don't see nearly as often.
If you plan to do drag only or really want to go cheap the 2011 Mustang GT is a great buy and can go very quick with basic mods.
Remember if you don't pay INVOICE price or BELOW on either of them you are screwing yourself. To many DEALS on all of these cars in the current market to pay even close to MSRP! Hell, you can get new ZR1's in the $90k's down here. Most dealers are offering $12k off plus the $3k GM cash rebate.
For the new GT500/used Z06 price, you can pick up a used 08+ Viper if you want something more different...