If only for the advances in tire technology, the 07 GT is the vastly superior car.
Assuming both cars are running on modern tires, the vintage muscle car might be better in a quarter mile drag race (some of those things were pumped up to well over 500HP), but the minute you need to stop or turn, the modern Mustang will annihilate the old stuff. Ditto for top speed: the better suspension and aerodynamics on the new 'stang will crush the old machines for top speed (and handling at very high speed).
Which is cooler, however, is open to debate. It's hard not to just grin evilly when you see a mint-condition old hemi 'cuda, Camaro SS or Shelby GT. Personally, I'd rather have the beautiful old retro ride than a modern car that might be far faster: A 70's Mach 1 is way, way cooler than a 2010 Nissan 370Z, even though it wouldn't have a prayer if they raced each other.
The '07 Mustang. Talk to someone who was driving in the 60's and didn't take drugs. My old man had a '65 Mustang 289 2bbl, new it ran 16's in the 1/4. His buddy had the hi-po '65 and it ran mid 15's. The same buddy had a '68 GT500KR and an Cobra Jet Fairlane, Dad said the Fairlane was the quickest... and the best they (dad and his friend) could personally get out of it was 13.9 @103. The 60's muscle cars had awful brakes and terrible handling.
Hell, I have a 1989 article from Hot Rod that compared the '89 Mustang GT, to the '70 Boss 302 and the 64 1/2 260. The '89 stopped shorter, handled better, out accelerated 0-60 & 1/4 mile and had a higher top speed than the other cars.
I have a '91 Mustang LX and while I love it, the handling and braking is terrible when compared to even a modern commuter box like the Focus. Time marches on.
You get a better more driveable package with a new performance car, just pick something with a large aftermarket following.
The only thing the old cars beat the new cars is in looks.
street legal you may be able to make the 67- 73 faster because of the lack of emission regulations on these older cars. of coarse they've improved immensely on handling and suspension and the newer cars are a bit lighter. both can be made very fast. in stock form the newer would edge out the older one.(but not my '68 gt500e convertible with a stroker 427ci )
Faster would be a 67-71 Muscle car, but it depends on the muscle car. The fastest stock muscle car ran a 12.9 on crappy tires back in 69. It was the 69 1/2 Plymouth Roadrunner 440 6pack. They are great cars, easy to work on, cheap to fix and reliable if built right, muscle cars that is.