The Subaru is a better all round car. The new Mustang is super sweet, but it's a muscle car. That's it. You've driven a RWD car in the snow before, right? It's horrible. The Subaru (speaking of personal experience) is a more comfortable, more reliable, and more practice vehicle. I love the STi personally, and I would get that car over just about any other car. You can take them anywhere, since they are rally cars, and they are very very dependable. Plus they hold their retail very well. And as much as I love the 5.0, I think the Subaru is a much better sports car, while the Mustang is a great Muscle cat.
Either way, it's gonna be a lot of fun. But if you get the stang, don't punk out and get the automatic.