This, coming from a die-hard muscle car guy, I would take the Sti over the Shelby Mustang. I have owned a number of high performance cars including two 427 Corvette roadsters and have driven or ridden in countless classic muscle cars. There is no disputing that muscle cars can be made to go fast in a straight line. They also have the sound and charisma often lacking in other high performance cars.
That said, the Sti is infinitely more refined than the Shelby. It was not only faster in 0-60 and the 1/4 mile, but its handling prowess on other worldly relative to the Shelby. The slalom time put down by the Sti would make it a very nice street machine indeed. Factor in the considerably superior braking as well as the awd which has a distince advantage in all weather and surface conditions, the Sti is a clear winner in my book.
The one downside to the Sti, which is a huge one indeed, is the absolutely atrocious styling. What was Subaru thinking? The car is just plain hideous IMO. The Shelby easily trumps the Sti in the styling department. The Shelby has that timeless, classic muscle car look that will still look good many years down the road while the Sti in 2012 will be passe. I always feel funny when I have to learn to love the styling of a car. A car's styling should "pop" out at you and get your heart beating fast right from the get go.
If we are at all honest, the Shelby Mustang is made for aging baby boomers, of which I am one. I am 55 and have duly noted that the gray hair sided boomers are buying and driving most the of the Shelby stuff. I think it is because they want to re-live their past or buy something they couldn't afford when they are younger.
For me, if I had the coin, I would buy one of each. Either that, or pray that Shelby stylists could go to work for Subaru. I will take high technology and rally type performance over a car that can lay down two black stripes on the asphalt but roll like an oil tanker in heavy seas. Subaru Sti gets my vote.