They're both the exact same car. Carroll Shelby makes a performance package for Ford Motors which is the "Shelby Mustang". There is no difference between the GT500 and the GT500KR when it comes to power and performance. They both have the 5.4 L/540 hp/510 lbf·ft supercharged DOHC 32 valve V8 Engine. The only difference is the addition of the "KR" package option. The "KR" stands for "King of the Road" and refers to the 1968 Shelby Mustang GT500KR. The 2007/2008 GT500KR has everything that the GT500 has but also includes a lighter Carbon Fiber hood w/hood pins, special "King of the Road" badging and racing stripes.

Now the question is, whether you want racing stripes and a carbon fiber hood because that's pretty much the only difference.